Positive electrode active material particle

ABSTRACT

A positive electrode active material particle with little deterioration is provided. A power storage device with little deterioration is provided. A highly safe power storage device is provided. The positive electrode active material particle includes a first crystal grain, a second crystal grain, and a crystal grain boundary positioned between the crystal grain and the second crystal grain; the first crystal grain and the second crystal grain include lithium, a transition metal, and oxygen; the crystal grain boundary includes magnesium and oxygen; and the positive electrode active material particle includes a region where the ratio of the atomic concentration of magnesium in the crystal grain boundary to the atomic concentration of the transition metal in first crystal grain and the second crystal grain is greater than or equal to 0.010 and less than or equal to 0.50.

TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method,or a manufacturing method. One embodiment of the present inventionrelates to a process, a machine, manufacture, or a composition ofmatter. One embodiment of the present invention relates to amanufacturing method of a semiconductor device, a display device, alight-emitting device, a power storage device, a lighting device, or anelectronic device. In particular, one embodiment of the presentinvention relates to a positive electrode active material that can beused in a secondary battery, a secondary battery, and an electronicdevice including a secondary battery.

Note that in this specification, the power storage device is acollective term describing units and devices having a power storagefunction. For example, a storage battery (also referred to as asecondary battery) such as a lithium-ion secondary battery, alithium-ion capacitor, and an electric double layer capacitor areincluded in the category of the power storage device.

Note that electronic devices in this specification mean all devicesincluding power storage devices, and electro-optical devices includingpower storage devices, information terminal devices including powerstorage devices, and the like are all electronic devices.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, a demand for lithium-ion secondarybatteries with high output and high capacity has rapidly grown with thedevelopment of the semiconductor industry, for portable informationterminals such as mobile phones, smartphones, and laptop computers;portable music players; digital cameras; medical equipment;next-generation clean energy vehicles such as hybrid electric vehicles(HEV), electric vehicles (EV), and plug-in hybrid electric vehicles(PHEV); and the like. The lithium-ion secondary batteries are essentialas rechargeable energy supply sources for today's information society.

Thus, improvement of a positive electrode active material has beenstudied to increase the cycle characteristics and the capacity of thelithium-ion secondary battery (Patent Document 1 and Patent Document 2).

The performance currently required for power storage devices includessafe operation under a variety of environments and longer-termreliability.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Published Patent Application No.2012-018914

[Patent Document 2] Japanese Published Patent Application No.2016-076454

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Lithium-ion secondary batteries and positive electrode active materialsused therein need an improvement in terms of capacity, cyclecharacteristics, charge and discharge characteristics, reliability,safety, cost, and the like.

In view of the above, an object of one embodiment of the presentinvention is to provide a positive electrode active material particlewith little deterioration. Another object of one embodiment of thepresent invention is to provide a novel positive electrode activematerial particle. Another object of one embodiment of the presentinvention is to provide a power storage device with littledeterioration. Another object of one embodiment of the present inventionis to provide a highly safe power storage device. Another object of oneembodiment of the present invention is to provide a novel power storagedevice.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects can bederived from the description of the specification, the drawings, and theclaims.

Means for Solving the Problems

One embodiment of the present invention is a positive electrode activematerial particle including a first crystal grain, a second crystalgrain, and a crystal grain boundary positioned between the first crystalgrain and the second crystal grain; the first crystal grain and thesecond crystal grain include lithium, a transition metal, and oxygen;and the crystal grain boundary includes magnesium and oxygen.

The above positive electrode active material particle preferablyincludes a region in which the ratio of the atomic concentration ofmagnesium to the atomic concentration of the transition metal is greaterthan or equal to 0.010 and less than or equal to 0.50.

In the above positive electrode active material particle, the crystalgrain boundary preferably further includes fluorine.

The above positive electrode active material particle preferablyincludes a region in which the ratio of the atomic concentration offluorine to the atomic concentration of the transition metal is greaterthan or equal to 0.020 and less than or equal to 1.00.

The above positive electrode active material particle preferablyincludes any one or more of iron, cobalt, nickel, manganese, chromium,titanium, vanadium, and niobium as the transition metal.

Effect of the Invention

According to one embodiment of the present invention, a positiveelectrode active material particle with little deterioration can beprovided. A novel positive electrode active material particle can beprovided. A power storage device with little deterioration can beprovided. A highly safe power storage device can be provided. A novelpower storage device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C Diagrams showing an example of a positive electrodeactive material particle.

FIGS. 2A to 2C Diagrams showing the concentration distribution in thepositive electrode active material particle.

FIG. 3 A diagram showing an example of a manufacturing method of thepositive electrode active material particle.

FIGS. 4A and 4B Cross-sectional views of an active material layer usinga graphene compound as a conductive additive.

FIGS. 5A and 5B Diagrams illustrating a coin-type secondary battery.

FIGS. 6A to 6D Diagrams illustrating a cylindrical secondary battery.

FIGS. 7A and 7B Diagrams illustrating an example of a secondary battery.

FIGS. 8A1, 8A2, 8B1, and 8B2 Diagrams illustrating examples of secondarybatteries.

FIGS. 9A and 9B Diagrams illustrating an example of a secondary battery.

FIGS. 10A and 10B Diagrams illustrating an example of a secondarybattery.

FIG. 11 A diagram illustrating an example of a secondary battery.

FIGS. 12A to 12C Diagrams illustrating a laminated secondary battery.

FIGS. 13A and 13B Diagrams illustrating a laminated secondary battery.

FIG. 14 An external view of a secondary battery.

FIG. 15 An external view of a secondary battery.

FIGS. 16A to 16C Diagrams illustrating a manufacturing method of asecondary battery.

FIGS. 17A, 17B1, 17B2, 17C, and 17D Diagrams illustrating a bendablesecondary battery.

FIGS. 18A and 18B Diagrams illustrating a bendable secondary battery.

FIGS. 19A to 19G Diagrams illustrating examples of electronic devices.

FIGS. 20A to 20C Diagrams illustrating an example of an electronicdevice.

FIG. 21 Diagram illustrating examples of electronic devices.

FIGS. 22A to 22C Diagrams illustrating examples of vehicles.

FIGS. 23A and 23B A cross-sectional TEM image and a schematic diagram ofa positive electrode active material particle according to Example.

FIGS. 24A and 24B Cross-sectional STEM images of a positive electrodeactive material particle according to Example.

FIG. 25 A view showing a HAADF-STEM image and an EDX point analysis of apositive electrode active material particle according to Example.

FIG. 26 A graph showing the EDX spectrum and the quantification resultsof a positive electrode active material particle according to Example.

FIG. 27 A graph showing the EDX spectrum and the quantification resultsof a positive electrode active material particle according to Example.

FIG. 28 A graph showing the EDX spectrum and the quantification resultsof a positive electrode active material particle according to Example.

FIG. 29 A graph showing the EDX spectrum and the quantification resultsof a positive electrode active material particle according to Example.

FIG. 30 A graph showing the EDX spectrum and the quantification resultsof a positive electrode active material particle according to Example.

FIGS. 31A to 31F Mapping images in EDX plane analysis of a positiveelectrode active material particle according to Example.

FIGS. 32A to 32F Mapping images in the EDX plane analysis of thepositive electrode active material particle according to Example.

FIGS. 33A and 33B Views showing EDX linear analysis of a positiveelectrode active material particle according to Example.

FIGS. 34A to 34F Graphs showing atomic concentrations in the EDX linearanalysis of the positive electrode active material particle according toExample.

FIGS. 35A to 35F Graphs showing atomic concentrations in the EDX linearanalysis of the positive electrode active material particle according toExample.

FIGS. 36A to 36D Graphs showing the ratio of atomic numbers in the EDXlinear analysis of the positive electrode active material particleaccording to Example.

FIGS. 37A to 37F Mapping images in EDX plane analysis of a positiveelectrode active material particle according to Example.

FIGS. 38A to 38F Mapping images in EDX plane analysis of a positiveelectrode active material particle according to Example.

FIGS. 39A to 39F Graphs showing atomic concentrations in EDX linearanalysis of a positive electrode active material particle according toExample.

FIGS. 40A to 40F Graphs showing atomic concentrations in the EDX linearanalysis of the positive electrode active material particle according toExample.

FIGS. 41A to 41D Graphs showing the ratio of atomic numbers in the EDXlinear analysis of the positive electrode active material particleaccording to Example.

FIGS. 42A and 42B A cross-sectional TEM image and a schematic diagram ofa positive electrode active material particle according to Example.

FIGS. 43A and 43B Cross-sectional STEM images of a positive electrodeactive material particle according to Example.

FIGS. 44A to 44F Mapping images in EDX plane analysis of a positiveelectrode active material particle according to Example.

FIGS. 45A to 45D Mapping images in the EDX plane analysis of thepositive electrode active material particle according to Example.

FIGS. 46A and 46B Views showing EDX linear analysis of a positiveelectrode active material particle according to Example.

FIGS. 47A to 47F Graphs showing atomic concentrations in the EDX linearanalysis of the positive electrode active material particle according toExample.

FIGS. 48A to 48D Graphs showing atomic concentrations in the EDX linearanalysis of the positive electrode active material particle according toExample.

FIGS. 49A to 49D Graphs showing the ratio of atomic numbers in the EDXlinear analysis of the positive electrode active material particleaccording to Example.

FIGS. 50A to 50F Mapping images in EDX plane analysis of a positiveelectrode active material particle according to Example.

FIGS. 51A to 51D Mapping images in the EDX plane analysis of thepositive electrode active material particle according to Example.

FIGS. 52A to 52F Graphs showing atomic concentrations in EDX linearanalysis of a positive electrode active material particle according toExample.

FIGS. 53A to 53D Graphs showing atomic concentrations in the EDX linearanalysis of the positive electrode active material particle according toExample.

FIGs. 54A to 54D Graphs showing the ratio of atomic numbers in the EDXlinear analysis of the positive electrode active material particleaccording to Example.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to drawings. Note that the present invention isnot limited to the description below, and it is easily understood bythose skilled in the art that modes and details of the present inventioncan be modified in various ways. In addition, the present inventionshould not be construed as being limited to the description in theembodiments given below.

Note that in drawings used in this specification, the sizes,thicknesses, and the like of components such as a positive electrode, anegative electrode, an active material layer, a separator, and anexterior body are exaggerated for simplicity in some cases. Therefore,the sizes of the components are not limited to the sizes in the drawingsand relative sizes between the components.

In structures of the present invention described in this specificationand the like, the same portions or portions having similar functions aredenoted by common reference numerals in different drawings, and thedescription thereof is not repeated. Further, the same hatching patternis applied to portions having similar functions, and the portions arenot especially denoted by reference numerals in some cases.

In the crystallography, a bar is placed over a number in the expressionof crystal planes and orientations; however, in this specification andthe like, crystal planes and orientations are expressed by placing aminus sign (−) at the front of a number because of expressionlimitations. Furthermore, an individual direction which shows anorientation in crystal is denoted by “[ ]”, a set direction which showsall of the equivalent orientations is denoted by “< >”, an individualplane which shows a crystal plane is denoted by “( )”, and a set planehaving equivalent symmetry is denoted by “{ }”.

In this specification and the like, segregation refers to a phenomenonin which, in a solid including a plurality of elements (e.g., A, B, andC), the concentration of a certain element (for example, B) isnon-uniformly distributed.

Embodiment 1

[Structure of Positive Electrode Active Material]

A positive electrode active material particle 100, which is oneembodiment of the present invention, is described with reference to FIG.1(A) to FIG. 1(C) and FIG. 2(A) to FIG. 2(C).

FIG. 1(A) illustrates an external view of the positive electrode activematerial particle 100. The positive electrode active material particle100 is an irregular particle. Note that the shape of the positiveelectrode active material particle 100 illustrated in FIG. 1(A) is anexample and not limited thereto.

The positive electrode active material particle 100 includes a pluralityof crystal grains 101 and a plurality of crystal grain boundaries 103.FIG. 1(B) illustrates the crystal grains 101 and the crystal grainboundaries 103 included in the positive electrode active materialparticle 100. The crystal grain boundaries 103 are denoted by dashedlines in FIG. 1(B); however, the boundary between the crystal grains 101and the crystal grain boundaries 103 may not be clear. Note that theshape and the number of the crystal grains 101 and the crystal grainboundaries 103 illustrated in FIG. 1(B) are examples and not limitedthereto.

The crystal grains 101 are particles each having a substantially uniformcrystal orientation. Adjacent crystal grains 101 each have a differentcrystal orientation and the crystal grain boundary 103 is between theadjacent crystal grains. That is, the positive electrode active materialparticle 100 includes a plurality of crystal grains 101 with the crystalgrain boundary 103 therebetween. The positive electrode active materialparticle 100 can also be referred to as a polycrystal. The positiveelectrode active material particle 100 may have a crystal defect 105 andmay include an amorphous region. Note that in this specification and thelike, a crystal defect refers to a body defect, a plane defect, or apoint defect which can be observed from a TEM image and the like, astructure in which another element enters the crystal, or the like. Notethat the crystal grain is referred to as a crystallite in some cases.

The crystal grains 101 and the crystal grain boundaries 103 in thepositive electrode active material particle 100 can be confirmed byX-ray diffraction (XRD), neutron diffraction, electron diffraction (ED),a transmission electron microscope (TEM) image, a scanning transmissionelectron microscopy (STEM) image, analysis of fast Fouriertransformation (FFT) performed on a lattice image obtained by the TEMimage or the STEM image, a high-angle annular dark field scanning TEM(HAADF-STEM) image, an annular bright-field scanning TEM (ABF-STEM)image, Raman spectroscopy, electron backscatter diffraction (EBSD), andthe like. Note that the electron backscatter diffraction is referred toas an electron backscatter diffraction pattern (EBSP) in some cases. Forexample, when the concentration (luminance) of a TEM image issubstantially uniform, the TEM image can be determined to have asubstantially uniform crystal orientation, i.e., to be a single crystalin some cases. Since the concentration (luminance) of a TEM imagechanges with crystal orientation, a region where the concentration(luminance) varies is regarded as a grain boundary in some cases.However, the clear boundary between the crystal grain 101 and thecrystal grain boundary 103 is not necessarily observed by the variousanalysis.

The crystal grain 101 and the crystal grain boundary 103 have differentcompositions. The crystal grain 101 includes lithium, a transitionmetal, and oxygen. The crystal grain boundary 103 includes magnesium andoxygen. The crystal grain boundary 103 preferably further includesfluorine.

The different compositions of the crystal grain 101 and the crystalgrain boundary 103 can be confirmed by energy dispersive X-rayspectroscopy (EDX), time-of-flight secondary ion mass spectrometry(ToF-SIMS), X-ray photoelectron spectroscopy (XPS), Auger electronspectroscopy (AES), electron energy-loss spectroscopy (EELS), and thelike. However, the clear boundary between the crystal grain 101 and thecrystal grain boundary 103 is not necessarily observed by the variousanalysis. A desired analysis target element may not be detected by someanalysis methods. The analysis target element may not be detected whenhaving an extremely low concentration.

<Crystal Grain Boundary>

The crystal grain boundary 103 included in the positive electrode activematerial particle 100 of one embodiment of the present inventionincludes magnesium and oxygen. The crystal grain boundary 103 includesmagnesium oxide. The crystal grain boundary 103 preferably furtherincludes fluorine. Fluorine may be substituted for part of oxygenincluded in magnesium oxide. Substitution of fluorine for part ofmagnesium oxide promotes diffusion of lithium, for example, so thatcharge and discharge are not prevented. The crystal grain boundary 103including fluorine is unlikely to dissolve in hydrofluoric acid in somecases.

The crystal grain boundary 103 includes a region with a higher magnesiumconcentration than the crystal grain 101. In other words, the crystalgrain boundary 103 includes a region where magnesium is segregated.

The crystal grain boundary 103 includes a region where the fluorineconcentration is higher than that in the crystal grain 101. In otherwords, the crystal grain boundary 103 includes a region where fluorineis segregated.

FIG. 2(B) and FIG. 2(C) respectively show an example of the magnesiumconcentration distribution and an example of the fluorine concentrationdistribution along the dashed-dotted line A1-A2 of the positiveelectrode active material particle 100 illustrated in FIG. 2(A). In FIG.2(B) and FIG. 2(C), the horizontal axis represents the distance of thedashed-dotted line A1-A2 in FIG. 2(A), and the vertical axis representsthe magnesium concentration (Mg Concentration) and the fluorineconcentration (F Concentration).

The crystal grain boundary 103 and the periphery of the crystal grainboundary 103 include a region where the concentrations of fluorine andmagnesium are higher than those in the crystal grain 101. The crystaldefect 105 also includes a region with high concentrations of magnesiumand fluorine in some cases. Note that in FIG. 2(B) and FIG. 2(C), thecrystal grain boundary 103 has, but is not limited to, the sameconcentration as that of the crystal defect 105. The shapes of themagnesium and fluorine concentration distributions are not limited tothose illustrated in FIG. 2(B) and FIG. 2(C).

Here, the number of transition metal atoms in the crystal grain 101 isdenoted as Tr-Metal. The number of transition metal atoms in the crystalgrain 101 (Tr-Metal) refers to the total number of atoms of eachtransition metal included in the crystal grain 101.

The positive electrode active material particle 100 preferably includesa region where the ratio of the number of magnesium atoms in the crystalgrain boundary 103 to the number of transition metal atoms in thecrystal grain 101 (Mg/Tr-Metal) is greater than or equal to 0.010 andless than or equal to 0.50. Further preferably, the positive electrodeactive material particle 100 includes a region where the Mg/Tr-Metal isgreater than or equal to 0.020 and less than or equal to 0.30. Stillfurther preferably, the positive electrode active material particle 100includes a region where the Mg/Tr-Metal is greater than or equal to0.030 and less than or equal to 0.20. The Mg/Tr-Metal in the aboveranges contributes to a reduction in deterioration of the positiveelectrode active material. That is, deterioration of the power storagedevice can be inhibited. In addition, a highly safe power storage devicecan be achieved.

Note that in this specification and the like, the transition metalrefers to an element belonging to Group 3 to Group 12 in the periodictable. The group numbers are based on the periodic table includingclassification of the first to 18^(th) groups, which is defined byInternational Union of Pure and Applied Chemistry (IUPAC) nomenclatureof inorganic chemistry (revision 1989).

In general, the repetition of charge and discharge of a power storagedevice causes the following side reactions: dissolution of a transitionmetal such as cobalt and manganese from a positive electrode activematerial particle included in the power storage device into anelectrolyte solution, release of oxygen, and an unstable crystalstructure, such that deterioration of the positive electrode activematerial particle proceeds in some cases. The deterioration of thepositive electrode active material particle might reduce the capacity ofthe power storage device, for example, thereby promoting thedeterioration of the power storage device. Note that in thisspecification and the like, a chemical or structural change of thepositive electrode active material particle, such as dissolution of atransition metal from a positive electrode active material particle intoan electrolyte solution, release of oxygen, and an unstable crystalstructure, is referred to as deterioration of the positive electrodeactive material particle in some cases. In this specification and thelike, a decrease in the capacity of the power storage device is referredto as deterioration of the power storage device in some cases.

A metal dissolved from the positive electrode active material particleis reduced at a negative electrode and precipitated, which might inhibitthe electrode reaction of the negative electrode. The precipitation ofthe metal in the negative electrode promotes deterioration such as adecrease in capacity in some cases.

A crystal lattice of the positive electrode active material particleexpands and contracts with insertion and extraction of lithium due tocharge and discharge, thereby undergoing strain and a change in volumein some cases. The strain and change in volume of the crystal latticecause cracking of the positive electrode active material particle, whichmight promote deterioration such as a decrease in capacity. The crackingof the positive electrode active material particle originates from acrystal grain boundary in some cases.

When the temperature within the power storage device turns high andoxygen is released from the positive electrode active material particle,the safety of the power storage device might be adversely affected. Inaddition, the release of oxygen might change the crystal structure ofthe positive electrode active material particle and promotedeterioration such as a decrease in capacity. Note that oxygen issometimes released from the positive electrode active material particleby insertion and extraction of lithium due to charge and discharge.

In contrast, magnesium oxide is a material with chemical and structuralstability. In a power storage device such as a lithium-ion secondarybattery, magnesium oxide itself included in a positive electrode activematerial particle is hardly involved in a battery reaction. That is,insertion and extraction of lithium hardly occur with magnesium oxide;thus, magnesium oxide itself is chemically and structurally stable evenafter charge and discharge.

The positive electrode active material particle 100 of one embodiment ofthe present invention, which includes magnesium oxide in the crystalgrain boundary 103, is chemically and structurally stable and hardlyundergoes a change in structure, a change in volume, and strain due tocharge and discharge. In other words, the crystal structure of thepositive electrode active material particle 100 is more stable andhardly changes even after repetition of charge and discharge. Inaddition, cracking of the positive electrode active material particle100 can be inhibited, which is preferable because deterioration such asa reduction in capacity can be reduced. When the charging voltageincreases and the amount of lithium in the positive electrode at thetime of charging decreases, the crystal structure becomes unstable andis more likely to deteriorate. The crystal structure of the positiveelectrode active material particle 100 of one embodiment of the presentinvention is particularly preferable because it is more stable and caninhibit deterioration such as a reduction in capacity.

Since the positive electrode active material particle 100 of oneembodiment of the present invention has a stable crystal structure,dissolution of a transition metal from the positive electrode activematerial particle can be inhibited, which is preferable becausedeterioration such as a reduction in capacity can be inhibited.

In the case where the positive electrode active material particle 100 ofone embodiment of the present invention is cracked along a crystal grainboundary, a surface of the positive electrode active material particleafter cracking includes magnesium oxide. In other words, a side reactioncan be inhibited even in the cracked positive electrode active materialand deterioration of the positive electrode active material can bereduced. That is, deterioration of the power storage device can beinhibited.

The positive electrode active material particle 100 of one embodiment ofthe present invention includes magnesium oxide in the crystal grainboundary 103, thereby inhibiting diffusion of oxygen included in thepositive electrode active material particle 100 through the crystalgrain boundary and suppressing release of oxygen from the positiveelectrode active material particle 100. The use of the positiveelectrode active material particle 100 can provide a highly safe powerstorage device.

In addition, the crystal defect 105 preferably includes magnesium oxidebecause the positive electrode active material particle 100 has a stablecrystal structure.

The positive electrode active material particle 100 preferably includesa region where the ratio of the number of fluorine atoms in the crystalgrain boundary 103 to the number of transition metal atoms in thecrystal grain 101 (F/Tr-Metal) is greater than or equal to 0.020 andless than or equal to 1.00. Further preferably, the positive electrodeactive material particle 100 includes a region where the F/Tr-Metal isgreater than or equal to 0.040 and less than or equal to 0.60. Stillfurther preferably, the positive electrode active material particle 100includes a region where the F/Tr-Metal is greater than or equal to 0.060and less than or equal to 0.40. The F/Tr-Metal in the above rangescontributes to efficient segregation of magnesium in the crystal grainboundary and the periphery thereof. That is, deterioration of thepositive electrode active material can be reduced. Deterioration of thepower storage device can be inhibited. In addition, a highly safe powerstorage device can be achieved.

<Crystal Grain>

The crystal grain 101 included in the positive electrode active materialparticle 100 of one embodiment of the present invention includeslithium, a transition metal, and oxygen. For example, the crystal grain101 includes a composite oxide containing lithium, a transition metal,and oxygen. As the transition metal, one or more of iron, cobalt,nickel, manganese, chromium, titanium, vanadium, and niobium can beused.

As the crystal grain 101, for example, a composite oxide with a layeredrock-salt crystal structure or a spinel crystal structure can be used.Alternatively, a polyanionic positive electrode material can be used asthe crystal grain 101. Examples of the polyanionic positive electrodematerial include a material with an olivine crystal structure and amaterial with a NASICON structure. Alternatively, a positive electrodematerial containing sulfur can be used as the crystal grain 101.

As the crystal grain 101, various composite oxides can be used. Forexample, a compound such as LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, Li₂MnO₃,V₂O₅, Cr₂O₅, or MnO₂ can be used.

As the material with a layered rock-salt crystal structure, for example,a composite oxide represented by LiMO₂ can be used. The element M ispreferably one or more elements selected from Co and Ni. LiCoO₂ ispreferable because it has high capacity, stability in the air, andthermal stability to a certain extent, for example. As the element M,one or more elements selected from Al and Mn may be included in additionto one or more elements selected from Co and Ni.

For example, it is possible to use LiNi_(x)Mn_(y)Co_(z)O_(w) (x, y, andz are each ⅓ or a neighborhood thereof and w is 2 or a neighborhoodthereof, for example). For example, it is possible to useLiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.8 or a neighborhood thereof, y is 0.1or a neighborhood thereof, z is 0.1 or a neighborhood thereof, and w is2 or a neighborhood thereof, for example). For example, it is possibleto use LiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.5 or a neighborhood thereof, yis 0.3 or a neighborhood thereof, z is 0.2 or a neighborhood thereof,and w is 2 or a neighborhood thereof, for example). For example, it ispossible to use LiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.6 or a neighborhoodthereof, y is 0.2 or a neighborhood thereof, z is 0.2 or a neighborhoodthereof, and w is 2 or a neighborhood thereof, for example). Forexample, it is possible to use LiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.4 or aneighborhood thereof, y is 0.4 or a neighborhood thereof, z is 0.2 or aneighborhood thereof, and w is 2 or a neighborhood thereof, forexample).

The neighborhood is, for example, a value greater than 0.9 times andsmaller than 1.1 times the predetermined value.

A material in which part of the transition metal and lithium included inthe crystal grain 101 is replaced with one or more elements selectedfrom Fe, Co, Ni, Cr, Al, Mg, and the like, or a material in which thecrystal grain 101 is doped with one or more elements selected from Fe,Co, Ni, Cr, Al, Mg, and the like may be used for the crystal grain 101.

As the material with a spinel crystal structure, for example, acomposite oxide represented by LiM₂O₄ can be used. It is preferable tocontain Mn as the element M. For example, LiMn₂O₄ can be used. It ispreferable to contain Ni in addition to Mn as the element M because thedischarge voltage and the energy density of the secondary battery areimproved in some cases. It is preferable to add a small amount oflithium nickel oxide (LiNiO₂ or LiNi_(1-x)M_(x)O₂ (M=Co, Al, or thelike)) to a lithium-containing material with a spinel crystal structurewhich contains manganese, such as LiMn₂O₄, because the characteristicsof the secondary battery can be improved.

The average diameter of primary particles of the positive electrodeactive material is preferably greater than or equal to 1 nm and lessthan or equal to 100 μm, further preferably greater than or equal to 50nm and less than or equal to 50 μm, and still further preferably greaterthan or equal to 1 μm and less than or equal to 30 μm, for example.Furthermore, the specific surface area is preferably greater than orequal to 1 m²/g and less than or equal to 20 m²/g. Furthermore, theaverage diameter of secondary particles is preferably greater than orequal to 5 μm and less than or equal to 50 μm. Note that the averageparticle diameters can be measured with a particle diameter distributionanalyzer or the like using a laser diffraction and scattering method orby observation with a scanning electron microscope (SEM) or a TEM. Thespecific surface area can be measured by a gas adsorption method.

A conductive material such as a carbon layer may be provided on thesurface of the positive electrode active material. With the conductivematerial such as the carbon layer, the conductivity of the electrode canbe increased. For example, the positive electrode active material can becoated with a carbon layer by mixing a carbohydrate such as glucose atthe time of baking the positive electrode active material. As theconductive material, graphene, multi-graphene, graphene oxide (GO), orreduced graphene oxide (RGO) can be used. Note that RGO refers to acompound obtained by reducing graphene oxide (GO), for example.

A layer containing one or more of an oxide and a fluoride may beprovided on a surface of the positive electrode active material. Theoxide may have a composition different from that of the crystal grain101. The oxide may have the same composition as the crystal grain 101.

As the polyanionic positive electrode material, for example, a compositeoxide containing oxygen, an element X, a metal A. and a metal M can beused. The metal M is one or more elements selected from Fe, Mn. Co, Ni.Ti, V, and Nb, the metal A is one or more elements selected from Li. Na.and Mg, and the element X is one or more elements selected from S, P,Mo, W, As, and Si.

As the material with an olivine crystal structure, for example, acomposite material (general formula LiMPO₄ (M is one or more of Fe(II),Mn(II), Co(II), and Ni(II))) can be used. Typical examples of thegeneral formula LiMPO₄ are lithium compounds such as LiFePO₄, LiNiPO₄,LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)CO_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1,and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(c)PO₄ (c+d+e<1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<, and0<i<1).

In particular, LiFePO₄ is preferable because it meets requirements withbalance for the positive electrode active material, such as safety,stability, high capacity density, and the existence of lithium ions thatcan be extracted in initial oxidation (charging).

The average diameter of primary particles of the positive electrodeactive material with an olivine crystal structure is preferably greaterthan or equal to 1 nm and less than or equal to 20 μm, furtherpreferably greater than or equal to 10 nm and less than or equal to 5μm, and still further preferably greater than or equal to 50 nm and lessthan or equal to 2 μm, for example. Furthermore, the specific surfacearea is preferably greater than or equal to 1 m²/g and less than orequal to 20 m²/g. Furthermore, the average diameter of secondaryparticles is preferably greater than or equal to 5 μm and less than orequal to 50 μm.

Alternatively, a composite material such as general formulaLi_((2-j))MSiO₄ (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II);0≤j≤2) may be used. Typical examples of the general formulaLi_((2-j))MSiO₄ are Li_((2-j))FeSiO₄, Li_((2-j))NiSiO₄,Li_((2-j))CoSiO₄, Li_((2-j))MnSiO₄, Li_((2-j))Fe_(k)Ni_(l)SiO₄,Li_((2-j))Fe_(k)Co_(l)SiO₄, Li_((2-j))Fe_(k)Mn_(l)SiO₄,Li_((2-j))Ni_(k)Co_(l)SiO₄, Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+1≤1, 0<k<1,and 0<l<1). Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound represented by a general formulaA_(x)M₂(XO₄)₃ (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, or Nb, X═S, P. Mo, W,As, or Si) can be used. Examples of the nasicon compound are Fe₂(MnO₄)₃,Fe₂(SO₄), and Li₃Fe₂(PO₄)₃. Further alternatively, a compoundrepresented by a general formula Li₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (M=Fe orMn) can be used as the crystal grain 101.

A perovskite fluoride such as NaFeF₃ and FeF₃, a metal chalcogenide (asulfide, a selenide, or a telluride) such as TiS₂ and MoS₂, an oxidewith an inverse spinel crystal structure such as LiMVO₄, a vanadiumoxide (V₂O₅, V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, an organicsulfur compound, or the like can be used as the crystal grain 101.

A borate-based positive electrode material represented by a generalformula LiMBO₃ (M is one or more of Fe(II), Mn(II), and Co(II)) can beused as the crystal grain 101.

As the crystal grain 101, for example, a solid solution obtained bycombining two or more composite oxides can be used. A solid solution ofLiMaO₂ and Li₂MbO₃ (M_(a) and M_(b) are independently one or moreelements selected from the transition metals) is referred to as alithium-excess oxide in some cases. For example, a solid solution ofLiNi_(x)Mn_(y)Co_(z)O₂ (x, y, z>0, x+y+z=1) and Li₂MnO₃ can be used asthe crystal grain 101.

As the crystal grain 101, a lithium-manganese composite oxiderepresented by a composition formula Li_(a)Mn_(b)M_(c)O_(d) can be used.Here, the element M is preferably a metal element other than lithium andmanganese, or silicon or phosphorus, further preferably nickel.Furthermore, in the case where the whole particle of a lithium-manganesecomposite oxide is measured, it is preferable to satisfy the followingat the time of discharging: 0<a/(b+c)<2; c>0; and 0.26≤(b+c)/d<0.5. Toachieve a high capacity, the surface portion and the middle portion ofthe lithium-manganese composite oxide preferably include regions withdifferent crystal structures, crystal orientations, or oxygen contents.In order that such a lithium-manganese composite oxide can be obtained,for example, 1.6≤a≤1.848, 0.19≤c/b≤0.935, and 2.5≤d≤3 are preferablysatisfied. Furthermore, it is particularly preferable to use alithium-manganese composite oxide represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃. In this specification and the like, alithium-manganese composite oxide represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃ refers to a lithium-manganese compositeoxide formed at a ratio (molar ratio) of the amounts of raw materials ofLi₂CO₃:MnCO₃:NiO=0.84:0.8062:0.318. Although this lithium-manganesecomposite oxide is represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃, the composition might deviate fromthis.

Note that the composition of metal, silicon, phosphorus, and otherelements in the whole particle of a lithium-manganese composite oxidecan be measured with, for example, an inductively coupled plasma massspectrometer (ICP-MS). The composition of oxygen in the whole particleof a lithium-manganese composite oxide can be measured by, for example,energy dispersive X-ray spectroscopy (EDX). Alternatively, thecomposition can be measured by ICP-MS combined with fusion gas analysisand valence evaluation of X-ray absorption fine structure (XAFS)analysis. Note that the lithium-manganese composite oxide is an oxidecontaining at least lithium and manganese, and may contain at least oneelement selected from chromium, cobalt, aluminum, nickel, iron,magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium,silicon, phosphorus, and the like.

Instead of lithium, sodium, potassium, strontium, barium, beryllium, orthe like may be used as carrier ions. For example, a sodium-containinglayered oxide can be used.

As the material containing sodium, for example, an oxide containingsodium, such as NaFeO₂, Na_(2/3)[Fe_(1/2)Mn_(1/2)]O₂,Na_(2/3)[Ni_(1/3)Mn_(2/3)]O₂, Na₂Fe₂(SO₄)₃, Na₃V₂(PO₄)₃, Na₂FePO₄F,NaVPO₄F, NaMPO₄ (M is one or more of Fe(II), Mn(II), Co(II), andNi(II)), Na₂FePO₄F, or Na₄Co₃(PO₄)₂P₂O₇, can be used.

As the positive electrode active material, a lithium-containing metalsulfide can be used. Examples of the lithium-containing metal sulfideare Li₂TiS₃ and Li₃NbS₄.

Although the example in which the positive electrode active materialparticle 100 includes the crystal grain 101 and the crystal grainboundary 103 has been described so far, one embodiment of the presentinvention is not limited thereto. For example, as illustrated in FIG.1(C), the positive electrode active material particle 100 may include aregion 107. The region 107 can be provided, for example, so as to be incontact with at least a part of the crystal grain 101. The region 107may be a coating film containing carbon such as graphene compounds ormay be a coating film containing lithium or an electrolyte decompositionproduct. When the region 107 is a coating film containing carbon, it ispossible to increase the conductivity between the positive electrodeactive particles 100 and between the positive electrode active materialparticle 100 and a current collector. In the case where the region 107is a coating film having decomposition products of lithium or anelectrolyte solution, over-reaction with the electrolyte solution can beinhibited, and cycle characteristics can be improved when used for asecondary battery.

When the particle size of the positive electrode active materialparticle 100 is too large, lithium diffusion is unlikely to occur. Incontrast, a too small particle size arises problems such as a reductionin the density of the electrode and over-reaction with an electrolytesolution. For these reasons, the particle size is preferably 1 μm ormore and 100 μm or less, further preferably 10 μm or more and 70 μm orless. Here, the particle size means a volume-based cumulative 50% value(D50), for example.

[Manufacturing Method of Positive Electrode Active Material]

A manufacturing method of the positive electrode active materialparticle 100 including the crystal grain 101 and the crystal grainboundary 103 is described with reference to FIG. 3. The crystal grain101 includes a composite oxide containing lithium, a transition metal(M), and oxygen. The crystal grain boundary 103 includes magnesium,fluorine, and oxygen.

First, starting materials are prepared (Step S11). Specifically, alithium source, a transition metal (M) source, a magnesium source, and afluorine source were individually weighed.

As the lithium source, for example, lithium carbonate, lithium fluoride,lithium hydroxide, or lithium oxide can be used.

As the transition metal (M) source, for example, one of more of a cobaltcompound, a nickel compound, a manganese compound, an iron compound, avanadium compound, a titanium compound, a molybdenum compound, a zinccompound, an indium compound, a gallium compound, a copper compound, aniobium compound, and the like can be used.

As the cobalt compound, for example, one or more of cobalt oxide, cobalthydroxide, cobalt oxyhydroxide, cobalt carbonate, cobalt oxalate, cobaltsulfate, and the like can be used.

As the nickel compound, for example, one or more of nickel oxide, nickelhydroxide, nickel carbonate, nickel chloride, nickel bromide, nickeliodide, nickel sulfate, nickel nitrate, nickel formate and the like canbe used.

As the manganese compound, for example, one or more of manganese oxide,manganese hydroxide, manganese carbonate, manganese chloride, manganeseiodide, manganese sulfate, manganese nitrate and the like can be used.

As the iron compound, for example, one or more of iron fluoride, ironchloride, iron bromide, iron iodide, iron sulfate, iron oxalate, ironacetate and the like can be used.

As the vanadium compound, for example, one or more of vanadium oxide,vanadium hydroxide, vanadium chloride, and vanadium sulfate, and thelike can be used.

As the titanium compound, for example, one or more of titanium fluoride,titanium chloride, titanium bromide, titanium iodide, titanium oxide,titanium sulfide, titanium sulfate, and the like can be used.

As the molybdenum compound, for example, one or more of molybdenumoxide, diammonium molybdate, phosphomolybdic acid, and the like can beused.

As the zinc compound, for example, one or more of zinc oxide, zinchydroxide, zinc nitrate, zinc sulfate, zinc chloride, zinc carbonate,and the like can be used.

As the indium compound, for example, one or more of indium chloride,indium sulfate, indium nitrate, indium oxide, indium hydroxide, and thelike can be used.

As the gallium compound, for example, one or more of gallium chloride,gallium fluoride, and the like can be used.

As the copper compound, for example, one or more of copper sulfate,copper chloride, copper nitrate, and the like can be used.

As the niobium compound, for example, one or more of niobium oxide,niobium chloride, niobium oxide sulfate, niobium fluoride, and the likecan be used.

As the magnesium source, for example, one or more of magnesium oxide,magnesium fluoride, magnesium hydroxide, magnesium carbonate, and thelike can be used.

As the fluorine source, for example, one or more of lithium fluoride andmagnesium fluoride can be used. That is, lithium fluoride can be used asboth a lithium source and a fluorine source, and magnesium fluoride canbe used as both a magnesium source and a fluorine source.

In the case where the crystal grain 101 includes the transition metal(M) and a metal other than the transition metal, the metal source otherthan the transition metal is weighed. In the case where aluminum isincluded as the metal other than the transition metal, an aluminumcompound can be used as the metal source, for example. As the aluminumcompound, one or more of aluminum oxide, aluminum hydroxide, aluminumcarbonate, aluminum chloride, aluminum iodide, aluminum sulfate,aluminum nitrate, and the like can be used.

The ratio between the number of transition metal (M) atoms and thenumber of magnesium atoms in the raw material is described. The ratio mof the number of magnesium atoms Mg(r) to the number of transition metal(M) atoms M(r) in the raw material is preferably greater than or equalto 0.0050 and less than or equal to 0.050, i.e., 0.0050≤m≤0.050 in thenumber of transition metal (M) atoms M(r): the number of magnesium atomsMg(r)=1.0:m. Furthermore, the ratio m of the number of magnesium atomsto the number of transition metal atoms is preferably 0.010 or aneighborhood thereof. With the above atomic ratio, the positiveelectrode active material including magnesium in the crystal grainboundary 103 can be produced effectively. Note that in the case where aplurality of kinds of transition metals are used as raw materials, thecalculation may be performed with the total number of atoms of theplurality of kinds of transition metals as the aforementioned number oftransition metal atoms M(r).

The neighborhood is, for example, a value greater than 0.9 times andsmaller than 1.1 times the predetermined value.

The ratio between the number of magnesium atoms and the number offluorine atoms in the raw material is described. The ratio n of thenumber of fluorine atoms F(r) to the number of magnesium atoms Mg(r) inthe raw material is preferably greater than or equal to 1.50 and lessthan or equal to 4.0, i.e., 1.50≤n≤4.0 in the number of magnesium atomsMg(r): the number of fluorine atoms F(r)=1.0:n. Furthermore, the ratio nof the number of fluorine atoms to the number of magnesium atoms ispreferably 2.0 or a neighborhood thereof. With the above atomic ratio,magnesium and fluorine can be segregated in the crystal grain boundary103 effectively.

The ratio among the atomic numbers of the transition metal, magnesium,and fluorine in the raw material can be represented by Formula 1. Here,m represents the ratio of the number of magnesium atoms Mg(r) to thenumber of transition metal atoms M(r). As described above,0.0050≤m≤0.050 is preferable and m=0.010 or a neighborhood thereof isfurther preferable. The ratio of the number of fluorine atoms F(r) tothe number of magnesium atoms Mg(r) is denoted by n. As described above,1.50<n<4.0 is preferable and n=2.0 or a neighborhood thereof is furtherpreferable.[Formula 1]M(r):Mg(r):F(r)=1.0: m:m×n  (1)

In the case where LiCoO₂ is fabricated as the positive electrode activematerial particle, the raw materials have the following ratio as anexample. The ratio m of the number of magnesium atoms to the number ofcobalt atoms is assumed to be 0.010. The ratio n of the number offluorine atoms to the number of magnesium atoms is assumed to be 2.0.According to Formula 1, the ratio among the atomic numbers of the rawmaterials, cobalt, magnesium, and fluorine can beCo:Mg:F=1.0:0.010:0.020.

Note that the aforementioned ratio of the atomic numbers of the rawmaterial does not always corresponds to the composition of the positiveelectrode active material particle 100 obtained by synthesis.

The molar ratio of the lithium compound and the transition metal (M)compound in the raw material may be a value corresponding to thecomposition of a presumed crystal grain. For example, in the case wherethe lithium composition of the obtained crystal grain is small relativeto the molar ratio of the lithium compound in the raw material, themolar ratio of the lithium compound in the raw material may beincreased.

Next, the weighed starting materials are mixed (Step S12). For example,a ball mill, a bead mill, or the like can be used for the mixing.

Next, a first heating is performed on the materials mixed in Step S12(Step S13). The first heating is preferably performed at higher than orequal to 800° C. and lower than or equal to 1050° C., further preferablyat higher than or equal to 900° C. and lower than or equal to 1000° C.The heating time is preferably greater than or equal to 2 hours and lessthan or equal to 20 hours. The first heating is preferably performed inan oxygen-containing atmosphere. For example, the first heating ispreferably performed in an atmosphere such as dry air.

By the first heating in Step S13, a composite oxide containing lithiumand a transition metal (M), that is included in the crystal grain 101,can be synthesized. Also, by the first heating, part of the magnesiumand fluorine contained in the starting material is segregated in thesuperficial portion of the composite oxide containing lithium and atransition metal (M). Note that another part of the magnesium andfluorine at this stage forms a solid solution in the composite oxidecontaining lithium and a transition metal (M).

Next, the material heated in Step S13 is cooled to room temperature(Step S14). After the cooling, the synthesized material is preferablysubjected to crushing treatment, in which case the size of the positiveelectrode active material particle 100 can be reduced.

Next, a second heating is performed on the material cooled in Step S14(Step S15). The second heating is preferably performed for a holdingtime at a specified temperature of 100 hours or shorter, furtherpreferably 1 hour or longer and 70 hours or shorter, further preferably2 hours or longer and 50 hours or shorter, and still further preferably2 hours or longer and 35 hours or shorter. The specified temperature ispreferably higher than or equal to 500° C. and lower than or equal to1200° C., further preferably higher than or equal to 700° C. and lowerthan or equal to 1000° C., and still further preferably about 800° C.The second heating is performed preferably in an oxygen-containingatmosphere. For example, the second heating is preferably performed inan atmosphere such as dry air.

The second heating in Step S15 promotes segregation of the magnesium andfluorine contained in the starting material on the crystal grainboundary.

Finally, the material heated in S15 is cooled to room temperature andcollected (Step S16), so that the positive electrode active materialparticle 100 can be obtained.

As described above, when the magnesium source and the fluorine sourceare mixed as the starting material, the positive electrode activematerial including magnesium oxide in the crystal grain boundary 103 canbe effectively fabricated.

Furthermore, when the magnesium source and the fluorine source are mixedas the starting material, magnesium is likely to be segregated in thecrystal grain boundary 103 in some cases.

When fluorine is substituted for oxygen bonded to magnesium, magnesiumeasily moves around the substituted fluorine in some cases.

Adding magnesium fluoride to magnesium oxide may lower the meltingpoint. When the melting point decreases, atoms are likely to move inheat treatment.

Fluorine has higher electronegativity than oxygen. Thus, even in astable compound such as magnesium oxide, when fluorine is added, unevencharge distribution occurs and thus a bond between magnesium and oxygenis weakened in some cases.

For these reasons, when the magnesium source and the fluorine source aremixed as the starting material, magnesium is likely to move and besegregated in the crystal grain boundary 103 in some cases.

By using the positive electrode active material particle 100 describedin this embodiment, a highly safe secondary battery with littledeterioration can be provided. This embodiment can be implemented inappropriate combination with any of the other embodiments.

Embodiment 2

In this embodiment, examples of materials which can be used for asecondary battery including the positive electrode active materialparticle 100 described in the above embodiment are described. In thisembodiment, a secondary battery in which a positive electrode, anegative electrode, and an electrolyte solution are wrapped in anexterior body is described as an example.

[Positive Electrode]

The positive electrode includes a positive electrode active materiallayer and a positive electrode current collector.

<Positive Electrode Active Material Layer>

The positive electrode active material layer includes a positiveelectrode active material particle. The positive electrode activematerial layer may contain a conductive additive and a binder.

As the positive electrode active material particle, the positiveelectrode active material particle 100 described in the above embodimentcan be used. When the above-described positive electrode active materialparticle 100 is used, a highly safe secondary battery with littledeterioration can be obtained.

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive with respect to the total amount of the activematerial layer is preferably greater than or equal to 1 wt % and lessthan or equal to 10 wt %, further preferably greater than or equal to 1wt % and less than or equal to 5 wt %.

A network for electric conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the positive electrode activematerial particles. The addition of the conductive additive to theactive material layer increases the electric conductivity of the activematerial layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber and isotropicpitch-based carbon fiber. In addition, carbon nanofiber, carbonnanotube, or the like can be used as carbon fiber. Carbon nanotube canbe formed by, for example, a vapor deposition method. Other examples ofthe conductive additive include carbon materials such as carbon black(e.g., acetylene black (AB)), graphite (black lead) particles, graphene,and fullerene. Alternatively, metal powder or metal fibers of copper,nickel, aluminum, silver, gold, or the like, a conductive ceramicmaterial, or the like can be used.

Alternatively, a graphene compound may be used as the conductiveadditive.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. Furthermore, a graphene compoundhas a planar shape. A graphene compound enables low-resistance surfacecontact. Furthermore, a graphene compound has extremely highconductivity even with a small thickness in some cases and thus allows aconductive path to be formed in an active material layer efficientlyeven with a small amount. For this reason, it is preferable to use agraphene compound as the conductive additive because the area where theactive material and the conductive additive are in contact with eachother can be increased or electric resistance can be reduced in somecases. Here, it is particularly preferable to use, for example,graphene, multilayer graphene, or reduced graphene oxide (hereinafter,RGO) as a graphene compound. Note that RGO refers to a compound obtainedby reducing graphene oxide (GO), for example.

In the case where an active material particle with a small particlediameter (e.g., 1 μm or less) is used, the specific surface area of theactive material particle is large and thus more conductive paths forconnecting the active material particles are needed. Thus, the amount ofconductive additive tends to increase and the supported amount of activematerial tends to decrease relatively. When the supported amount ofactive material decreases, the capacity of the secondary battery alsodecreases. In such a case, a graphene compound that can efficiently forma conductive path even in a small amount is particularly preferably usedas the conductive additive because the supported amount of activematerial does not decrease.

A cross-sectional structure example of an active material layer 200containing a graphene compound as a conductive additive is describedbelow.

FIG. 4(A) shows a longitudinal cross-sectional view of the activematerial layer 200. The active material layer 200 includes the positiveelectrode active material particle 100, a graphene compound 201 servingas a conductive additive, and a binder (not illustrated). Here, grapheneor multilayer graphene may be used as the graphene compound 201, forexample. The graphene compound 201 preferably has a sheet-like shape.The graphene compound 201 may have a sheet-like shape formed of aplurality of sheets of multilayer graphene and/or a plurality of sheetsof graphene that partly overlap with each other.

In the longitudinal cross section of the active material layer 200, asillustrated in FIG. 4(A), the sheet-like graphene compounds 201 aredispersed substantially uniformly in the active material layer 200. Thegraphene compounds 201 are schematically shown by thick lines in FIG.4(A) but are actually thin films each having a thickness correspondingto the thickness of a single layer or a multi-layer of carbon molecules.The plurality of graphene compounds 201 are formed in such a way as towrap or cover the plurality of positive electrode active materialparticles 100 or adhere to the surfaces of the plurality of positiveelectrode active material particles 100, so that the graphene compounds201 make surface contact with the positive electrode active materialparticles 100.

Here, when the plurality of graphene compounds are bonded to each other,a net-like graphene compound sheet (hereinafter referred to as agraphene compound net or a graphene net) can be formed. The graphene netcovering the active material can function as a binder for bonding activematerials. The amount of a binder can thus be reduced, or the binderdoes not have to be used, increasing the proportion of the activematerial in the electrode volume or weight. That is to say, the capacityof the power storage device can be increased.

Here, it is preferable that graphene oxide be used as the graphenecompounds 201 and mixed with an active material to form a layer to bethe active material layer 200, and then reduction be performed. Whengraphene oxide with extremely high dispersibility in a polar solvent isused for the formation of the graphene compounds 201, the graphenecompounds 201 can be substantially uniformly dispersed in the activematerial layer 200. The solvent is removed by volatilization from adispersion medium in which graphene oxide is uniformly dispersed, andthe graphene oxide is reduced, hence, the graphene compounds 201remaining in the active material layer 200 partly overlap with eachother and are dispersed such that surface contact is made, therebyforming a three-dimensional conductive path. Note that graphene oxidecan be reduced either by heat treatment or with the use of a reducingagent, for example.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenecompound 201 is capable of making low-resistance surface contact;accordingly, the electrical conduction between the positive electrodeactive material particles 100 and the graphene compound 201 can beimproved with a smaller amount of the graphene compound 201 than that ofa normal conductive additive. This increases the proportion of thepositive electrode active material particle 100 in the active materiallayer 200. Accordingly, the discharge capacity of the power storagedevice can be increased.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, or ethylene-propylene-diene copolymer can be used, forexample. Alternatively, fluororubber can be used as the binder.

As the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, a polysaccharide and the like can beused. As the polysaccharide, a cellulose derivative such ascarboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,starch, or the like can be used. It is more preferred that suchwater-soluble polymers be used in combination with any of the aboverubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF),polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinylacetate, or nitrocellulose is preferably used.

A plurality of the above materials may be used in combination for thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, a rubber material or the like is preferably mixed with a materialhaving a significant viscosity modifying effect, for example. As amaterial having a significant viscosity modifying effect, for example, awater-soluble polymer may be used. An example of a water-soluble polymerhaving an especially significant viscosity modifying effect is theabove-mentioned polysaccharide; for example, a cellulose derivative suchas carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and anothermaterial combined as a binder such as styrene-butadiene rubber in anaqueous solution. Furthermore, a water-soluble polymer is expected to beeasily and stably adsorbed to an active material surface because it hasa functional group. Many cellulose derivatives such as carboxymethylcellulose have functional groups such as a hydroxyl group and a carboxylgroup, and because of the functional groups, polymers are expected tointeract with each other and cover an active material surface in a largearea.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolytesolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolyte solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while inhibitingelectric conduction.

<Positive Electrode Current Collector>

For the positive electrode current collector, a material that has highconductivity, such as a metal like stainless steel, gold, platinum,aluminum, or titanium, or an alloy thereof, can be used. It is preferredthat a material used for the positive electrode current collector notdissolve at the potential of the positive electrode. It is also possibleto use an aluminum alloy to which an element that improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. The positive electrode current collector can alsobe formed with a metal element that forms silicide by reacting withsilicon. Examples of the metal element that forms silicide by reactingwith silicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. Thecurrent collector can have any of various shapes including a foil-likeshape, a plate-like shape (sheet-like shape), a net-like shape, apunching-metal shape, and an expanded-metal shape. The current collectorpreferably has a thickness of 5 μm to 30 μm.

[Negative Electrode]

The negative electrode includes a negative electrode active materiallayer and a negative electrode current collector. The negative electrodeactive material layer may contain a conductive additive and a binder.

<Negative Electrode Active Material>

As a negative electrode active material, for example, an alloy-basedmaterial or a carbon-based material can be used.

For the negative electrode active material, an element which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. For example, a material containing atleast one of silicon, tin, gallium, aluminum, germanium, lead, antimony,bismuth, silver, zinc, cadmium, indium, and the like can be used. Suchelements have higher capacity than carbon; in particular, silicon has ahigh theoretical capacity of 4200 mAh/g. For this reason, silicon ispreferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sns, Ag₃Sn, Ag₃Sb, NizMnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Here, an element thatenables charge-discharge reactions by an alloying reaction and adealloying reaction with lithium, a compound containing the element, andthe like may be referred to as an alloy-based material.

In this specification and the like. SiO refers, for example, to siliconmonoxide. SiO can alternatively be expressed as SiOx. Here, x ispreferably 1 or an approximate value of 1. For example, x is preferably0.2 or more and 1.5 or less, further preferably 0.3 or more and 1.2 orless.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), a carbon nanotube,graphene, carbon black, and the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it canrelatively easily have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.05 V and lower than or equal to 0.3 Vvs. Li/Li) when lithium ions are intercalated into the graphite (while alithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such as arelatively high capacity per unit volume, relatively small volumeexpansion, low cost, and higher level of safety than that of a lithiummetal.

Alternatively, for the negative electrode active material, an oxide suchas titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm^(t)).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active material. For example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used forthe negative electrode active material. Other examples of the materialwhich causes a conversion reaction include oxides such as Fe₂O₃, CuO,Cu₂O, RuO₂, and Cr₂O₃, sulfides such as CoS_(0.89), NiS, and CuS,nitrides such as Zn₃N₂, Cu N, and Ge₃N₄, phosphides such as NiP₂, FeP₂,and CoP₃, and fluorides such as FeF₃ and BiF₃.

For the conductive additive and the binder that can be included in thenegative electrode active material layer, materials similar to those ofthe conductive additive and the binder that can be included in thepositive electrode active material layer can be used.

<Negative Electrode Current Collector>

For the negative electrode current collector, a material similar to thatof the positive electrode current collector can be used. Note that amaterial that is not alloyed with a carrier ion such as lithium ispreferably used for the negative electrode current collector.

[Electrolyte Solution]

The electrolyte solution contains a solvent and an electrolyte. As asolvent of the electrolyte solution, an aprotic organic solvent ispreferably used, for example, one of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate, chloroethylene carbonate, vinylenecarbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate,methyl acetate, ethyl acetate, methyl propionate, ethyl propionate,propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

When a phosphoric ester compound containing fluorine or a carbonic estercompound containing fluorine, which has non-flammability, is used as asolvent of the electrolyte solution, a power storage device can beprevented from exploding or catching fire, for example. An example ofthe phosphoric ester compound containing fluorine istris(2,2,2-trifluoroethyl)phosphate (TFEP). An example of the carbonicester compound containing fluorine is bis(2,2,2-trifluoroethyl)carbonate(TFEC).

When a gelled high-molecular material is used as the solvent of theelectrolyte solution, safety against liquid leakage and the like isimproved. Furthermore, a secondary battery can be thinner and morelightweight. Typical examples of gelled high-molecular materials includea silicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, and a gel of afluorine-based polymer.

When one or more kinds of ionic liquids (room temperature molten salts)which have non-flammability and non-volatility is used as a solvent ofthe electrolyte solution, a power storage device can be prevented fromexploding or catching fire even when the power storage device internallyshorts out or the internal temperature increases owing to overchargingor the like. An ionic liquid is made with a cation and an anion, andcontains an organic cation and an anion.

Examples of the organic cation used for the electrolyte solution includealiphatic onium cations such as a quaternary ammonium cation, a tertiarysulfonium cation, and a quaternary phosphonium cation, and aromaticcations such as an imidazolium cation and a pyridinium cation. Examplesof the anion used for the electrolyte solution include a monovalentamide-based anion, a monovalent methide-based anion, a fluorosulfonateanion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, and aperfluoroalkylphosphate anion.

As an electrolyte dissolved in the above-described solvent, one oflithium salts such as LiPFb, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂BI₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃,LiC(CF₃SO₂)₃, LiC(C₂FSO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), andLiN(C₂FsSO₂)₂ can be used, or two or more of these lithium salts can beused in an appropriate combination in an appropriate ratio.

The electrolyte solution used for a storage device is preferably highlypurified and contains a small amount of dust particles and elementsother than the constituent elements of the electrolyte solution(hereinafter also simply referred to as impurities). Specifically, theweight ratio of impurities to the electrolyte solution is less than orequal to 1%, preferably less than or equal to 0.1%, and furtherpreferably less than or equal to 0.01%.

Furthermore, vinylene carbonate, propane sultone (PS), tert-butylbenzene(TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate(LiBOB), a dinitrile compound such as succinonitrile or adiponitrile,triisopropoxy boroxine (TiPBx), sulfolane, hydrofluoroether (HFE), vinylacetate (VA), or the like may be added to the electrolyte solution. Theconcentration of the added material is, for example, higher than orequal to 0.1 weight % and lower than or equal to 5 weight/with respectto the whole solvent.

Alternatively, a polymer gel electrolyte obtained in such a manner thata polymer is swelled with an electrolyte solution may be used.

When a polymer gel electrolyte is used, safety against liquid leakageand the like is improved. Furthermore, a secondary battery can bethinner and more lightweight.

As the gelled molecular, a silicone gel, an acrylic gel, anacrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, and a gel of a fluorine-based polymer can be used.Examples of the polymer include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP), can be used. Theformed polymer may be porous.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or the like, or a solid electrolyteincluding a high-molecular material such as a polyethylene oxide(PEO)-based high-molecular material, or the like may be used. When thesolid electrolyte is used, a separator and a spacer are not necessary.Furthermore, since the battery can be entirely solidified, there is nopossibility of liquid leakage to increase the safety of the batterydramatically.

[Separator]

The secondary battery preferably includes a separator. As the separator,for example, a fiber containing cellulose, such as paper; nonwovenfabric; a glass fiber; ceramics; a synthetic fiber containing nylon(polyamide), vinylon (polyvinyl alcohol-based fiber), polyester,acrylic, polyolefin, or polyurethane; or the like can be used. Theseparator is preferably formed to have an envelope-like shape to wrapone of the positive electrode and the negative electrode.

The separator may have a multilayer structure. For example, an organicmaterial film such as polypropylene or polyethylene can be coated with aceramic-based material, a fluorine-based material, a polyamide-basedmaterial, a mixture thereof, or the like. As the ceramic-based material,for example, aluminum oxide particles or silicon oxide particles can beused. As the fluorine-based material, for example, PVDF or apolytetrafluoroethylene can be used. As the polyamide-based material,for example, nylon or aramid (meta-based aramid or para-based aramid)can be used.

Oxidation resistance is improved when the separator is coated with theceramic-based material, so that deterioration of the separator incharging and discharging at high voltage can be inhibited and thus thereliability of the secondary battery can be improved. In addition, whenthe separator is coated with the fluorine-based material, the separatoris easily brought into close contact with an electrode, resulting inhigh output characteristics. When the separator is coated with thepolyamide-based material, in particular, aramid, heat resistance isimproved to increase the safety of the secondary battery.

For example, both surfaces of a polypropylene film may be coated with amixed material of aluminum oxide and aramid. Alternatively, a surface ofthe polypropylene film in contact with the positive electrode may becoated with the mixed material of aluminum oxide and aramid, and asurface of the polypropylene film in contact with the negative electrodemay be coated with the fluorine-based material.

With the use of a separator having a multilayer structure, the capacityof the secondary battery per volume can be increased because the safetyof the secondary battery can be maintained even when the total thicknessof the separator is small.

Embodiment 3

In this embodiment, examples of the shape of a secondary batteryincluding the positive electrode active material particle 100 describedin the above embodiments are described. For the materials used for thesecondary battery described in this embodiment, the description of theabove embodiments can be referred to.

[Coin-Type Secondary Battery]

First, an example of a coin-type secondary battery is described. FIG.5(A) is an external view of a coin-type (single-layer flat type)secondary battery, and FIG. 5(B) is a cross-sectional view thereof.

In a coin-type secondary battery 30X), a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type secondary battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel) can beused. Alternatively, the positive electrode can 301 and the negativeelectrode can 302 are preferably covered with nickel, aluminum, or thelike in order to prevent corrosion due to the electrolyte solution. Thepositive electrode can 301 and the negative electrode can 302 areelectrically connected to the positive electrode 304 and the negativeelectrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 5(B), the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303located therebetween. In such a manner, the coin-type secondary battery300 can be manufactured.

When the positive electrode active material particle 100 described inthe above embodiments is used in the positive electrode 304, thecoin-type secondary battery 300 with little deterioration and highsafety can be obtained.

[Cylindrical Secondary Battery]

An example of a cylindrical secondary battery will be described withreference to FIG. 6(A) to FIG. 6(D). A cylindrical secondary battery 600illustrated in FIG. 6(A) includes, as illustrated in the cross-sectionalschematic view of FIG. 6(B), a positive electrode cap (battery lid) 601on the top surface and a battery can (outer can) 602 on the side andbottom surfaces. The positive electrode cap and the battery can (outercan) 602 are insulated from each other by a gasket (insulating packing)610.

Inside the battery can 602 having a hollow cylindrical shape, a batteryelement in which a strip-like positive electrode 604 and a strip-likenegative electrode 606 are wound with a separator 605 locatedtherebetween is provided. Although not illustrated, the battery elementis wound around a center pin. One end of the battery can 602 is closeand the other end thereof is open. For the battery can 602, a metalhaving a corrosion-resistant property to an electrolyte solution, suchas nickel, aluminum, or titanium, an alloy of such a metal, or an alloyof such a metal and another metal (e.g., stainless steel) can be used.Alternatively, the battery can 602 is preferably covered with nickel,aluminum, or the like in order to prevent corrosion due to theelectrolyte solution. Inside the battery can 602, the battery element inwhich the positive electrode, the negative electrode, and the separatorare wound is provided between a pair of insulating plates 608 and 609that face each other. Furthermore, a nonaqueous electrolyte solution(not illustrated) is injected inside the battery can 602 provided withthe battery element. As the nonaqueous electrolyte solution, anonaqueous electrolyte solution that is similar to that of the coin-typesecondary battery can be used.

Since the positive electrode and the negative electrode of thecylindrical secondary battery are wound, active materials are preferablyformed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery increases to over a predeterminedthreshold value. The PTC element 611, which serves as a thermallysensitive resistor whose resistance increases as temperature rises,limits the amount of current by increasing the resistance, therebypreventing abnormal heat generation. Barium titanate (BaTiO₃)-basedsemiconductor ceramic or the like can be used for the PTC element.

Alternatively, as illustrated in FIG. 6(C), a plurality of secondarybatteries 600 may be sandwiched between a conductive plate 613 and aconductive plate 614 to form a module 615. The plurality of secondarybatteries 600 may be connected parallel to each other, connected inseries, or connected in series after being connected parallel to eachother. With the module 615 including the plurality of secondarybatteries 600, large electric power can be extracted.

FIG. 6(D) is a top view of the module 615. The conductive plate 613 isshown by a dotted line for clarity of the drawing. As illustrated inFIG. 6(D), the module 615 may include a wiring 616 which electricallyconnects the plurality of secondary batteries 600 to each other. It ispossible to provide the conductive plate 613 over the wiring 616 tooverlap with each other. In addition, a temperature control device 617may be provided between the plurality of secondary batteries 600. Whenthe secondary batteries 600 are overheated, the temperature controldevice 617 can cool them, and when the secondary batteries 600 arecooled too much, the temperature control device 617 can heat them. Thus,the performance of the module 615 is not easily influenced by theoutside air temperature.

When the positive electrode active material particle 100 described inthe above embodiments is used in the positive electrode 604, thecylindrical secondary battery 600 with little deterioration and highsafety can be obtained.

[Structural Examples of Power Storage Device]

Other structural examples of power storage devices will be describedwith reference to FIG. 7 to FIG. 11.

FIG. 7(A) and FIG. 7(B) are external views of a power storage device.The power storage device includes a circuit board 900 and a secondarybattery 913. A label 910 is attached onto the secondary battery 913. Thepower storage device further includes a terminal 951, a terminal 952, anantenna 914, and an antenna 915 as illustrated in FIG. 7(B).

The circuit board 900 includes a terminal 911 and a circuit 912. Theterminal 911 is connected to the terminal 951, the terminal 952, theantenna 914, the antenna 915, and the circuit 912. Note that a pluralityof terminals 911 serving as a control signal input terminal, a powersupply terminal, and the like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. Note that the shape of the antenna 914 and the antenna 915 is notlimited to a coil shape and may be a linear shape or a plate shape.Further, a planar antenna, an aperture antenna, a traveling-waveantenna, an EH antenna, a magnetic-field antenna, or a dielectricantenna may be used. The antenna 914 or the antenna 915 may be aflat-plate conductor. The flat-plate conductor can serve as one ofconductors for electric field coupling. That is, the antenna 914 or theantenna 915 can serve as one of two conductors of a capacitor. Thus,electric power can be transmitted and received not only by anelectromagnetic field or a magnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage device includes a layer 916 between the secondarybattery 913, and the antenna 914 and the antenna 915. The layer 916 hasa function of, for example, blocking an electromagnetic field from thesecondary battery 913. As the layer 916, for example, a magnetic bodycan be used.

Note that the structure of the power storage device is not limited tothat shown in FIG. 7.

For example, as shown in FIG. 8(A-1) and FIG. 8(A-2), two oppositesurfaces of the secondary battery 913 illustrated in FIG. 7(A) and FIG.7(B) may be provided with an antenna. FIG. 8(A-1) is an external viewshowing one side of the opposite surfaces, and FIG. 8(A-2) is anexternal view showing the other side of the opposite surfaces. Forportions similar to those illustrated in FIG. 7(A) and FIG. 7(B), adescription of the power storage device illustrated in FIG. 7(A) andFIG. 7(B) can be referred to as appropriate.

As illustrated in FIG. 8(A-1), the antenna 914 is provided on one of theopposite surfaces of the secondary battery 913 with the layer 916located therebetween, and as illustrated in FIG. 8(A-2), the antenna 915is provided on the other of the opposite surfaces of the secondarybattery 913 with a layer 917 located therebetween. The layer 917 has afunction of, for example, blocking an electromagnetic field from thesecondary battery 913. As the layer 917, for example, a magnetic bodycan be used.

With the above structure, both the antenna 914 and the antenna 915 canbe increased in size.

Alternatively, as illustrated in FIG. 8(B-1) and FIG. 8(B-2), twoopposite surfaces of the secondary battery 913 in FIG. 7(A) and FIG.7(B) may be provided with different types of antennas. FIG. 8(B-1) is anexternal view showing one side of the opposite surfaces, and FIG. 8(B-2)is an external view showing the other side of the opposite surfaces. Forportions similar to those in FIG. 7(A) and FIG. 7(B), a description ofthe power storage device illustrated in FIG. 7(A) and FIG. 7(B) can bereferred to as appropriate.

As illustrated in FIG. 8(B-1), the antenna 914 and the antenna 915 areprovided on one of the opposite surfaces of the secondary battery 913with the layer 916 interposed therebetween, and as illustrated in FIG.8(B-2), an antenna 918 is provided on the other of the opposite surfacesof the secondary battery 913 with the layer 917 interposed therebetween.The antenna 918 has a function of, for example, communicating data withan external device. An antenna with a shape that can be applied to theantenna 914 and the antenna 915, for example, can be used as the antenna918. As a system for communication using the antenna 918 between thepower storage device and another device, a response method that can beused between the power storage device and another device, such as NFC,can be employed.

Alternatively, as illustrated in FIG. 9(A), the secondary battery 913 inFIG. 7(A) and FIG. 7(B) may be provided with a display device 920. Thedisplay device 920 is electrically connected to the terminal 911 via aterminal 919. It is possible that the label 910 is not provided in aportion where the display device 920 is provided. For portions similarto those in FIG. 7(A) and FIG. 7(B), a description of the power storagedevice illustrated in FIG. 7(A) and FIG. 7(B) can be referred to asappropriate.

The display device 920 can display, for example, an image showingwhether charging is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescence (also referredto as EL) display device, or the like can be used. For example, the useof electronic paper can reduce the power consumption of the displaydevice 920.

Alternatively, as illustrated in FIG. 9(B), the secondary battery 913illustrated in FIG. 7(A) and FIG. 7(B) may be provided with a sensor921. The sensor 921 is electrically connected to the terminal 911 via aterminal 922. For portions similar to those illustrated in FIG. 7(A) andFIG. 7(B), a description of the power storage device illustrated in FIG.7(A) and FIG. 7(B) can be referred to as appropriate.

The sensor 921 has a function of measuring, for example, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, electric current, voltage,electric power, radiation, flow rate, humidity, gradient, oscillation,odor, or infrared rays. With the sensor 921, for example, data on anenvironment (e.g., temperature) where the power storage device is placedcan be sensed and stored in a memory inside the circuit 912.

Further structural examples of the secondary battery 913 will bedescribed with reference to FIG. 10 and FIG. 11.

The secondary battery 913 illustrated in FIG. 10(A) includes a woundbody 950 provided with the terminal 951 and the terminal 952 inside ahousing 930. The wound body 950 is soaked in an electrolyte solutioninside the housing 930. The terminal 952 is in contact with the housing930, and an insulator or the like inhibits contact between the terminal951 and the housing 930. Note that in FIG. 10(A), the housing 930divided into two pieces is illustrated for convenience; however, in theactual structure, the wound body 950 is covered with the housing 930 andthe terminal 951 and the terminal 952 extend to the outside of thehousing 930. For the housing 930, a metal material (such as aluminum) ora resin material can be used.

Note that as illustrated in FIG. 10(B), the housing 930 in FIG. 10(A)may be formed using a plurality of materials. For example, in thesecondary battery 913 in FIG. 10(B), a housing 930 a and a housing 930 bare bonded to each other, and the wound body 950 is provided in a regionsurrounded by the housing 930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, blocking of an electricfield from the secondary battery 913 can be inhibited. When an electricfield is not significantly blocked by the housing 930 a, an antenna suchas the antenna 914 and the antenna 915 may be provided inside thehousing 930 a. For the housing 930 b, a metal material can be used, forexample.

FIG. 11 illustrates the structure of the wound body 950. The wound body950 includes a negative electrode 931, a positive electrode 932, andseparators 933. The wound body 950 is obtained by winding a sheet of astack in which the negative electrode 931 overlaps with the positiveelectrode 932 with the separator 933 provided therebetween. Note that aplurality of stacks each including the negative electrode 931, thepositive electrode 932, and the separator 933 may be further stacked.

The negative electrode 931 is connected to the terminal 911 illustratedin FIG. 7 via one of the terminal 951 and the terminal 952. The positiveelectrode 932 is connected to the terminal 911 illustrated in FIG. 7 viathe other of the terminal 951 and the terminal 952.

When the positive electrode active material particle 100 described inthe above embodiments is used in the positive electrode 932, thesecondary battery 913 with little deterioration and high safety can beobtained.

[Laminated Secondary Battery]

Next, an example of a laminated secondary battery will be described withreference to FIG. 12 to FIG. 17. When the laminated secondary batteryhas flexibility and is used in an electronic device at least part ofwhich is flexible, the secondary battery can be bent as the electronicdevice is bent.

A laminated secondary battery 980 is described with reference to FIG.12. The laminated secondary battery 980 includes a wound body 993illustrated in FIG. 12(A). The wound body 993 includes a negativeelectrode 994, a positive electrode 995, and a separator 996.

The wound body 993 is, like the wound body 950 illustrated in FIG. 11,obtained by winding a sheet of a stack in which the negative electrode994 overlaps with the positive electrode 995 with the separator 996therebetween.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 may be determinedas appropriate depending on capacity and an element volume which arerequired. The negative electrode 994 is connected to a negativeelectrode current collector (not illustrated) via one of a leadelectrode 997 and a lead electrode 998, and the positive electrode 995is connected to a positive electrode current collector (not illustrated)via the other of the lead electrode 997 and the lead electrode 998.

As illustrated in FIG. 12(B), the wound body 993 is packed in a spaceformed by bonding a film 981 and a film 982 having a depressed portionthat serve as exterior bodies by thermocompression bonding or the like,whereby the secondary battery 980 can be formed as illustrated in FIG.12(C). The wound body 993 includes the lead electrode 997 and the leadelectrode 998, and is soaked in an electrolyte solution inside a spacesurrounded by the film 981 and the film 982 having a depressed portion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible storage battery can be fabricated.

Although FIG. 12(B) and FIG. 12(C) illustrate an example where a spaceis formed by two films, the wound body 993 may be placed in a spaceformed by bending one film.

When the positive electrode active material particle 100 described inthe above embodiments is used in the positive electrode 995, thesecondary battery 980 with little deterioration and high safety can beobtained.

In FIG. 12, an example in which the secondary battery 980 includes awound body in a space formed by films serving as exterior bodies isdescribed; however, as illustrated in FIG. 13, a secondary battery mayinclude a plurality of strip-shaped positive electrodes, a plurality ofstrip-shaped separators, and a plurality of strip-shaped negativeelectrodes in a space formed by films serving as exterior bodies, forexample.

A laminated secondary battery 500 illustrated in FIG. 13(A) includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isprovided between the positive electrode 503 and the negative electrode506 in the exterior body 509. The exterior body 509 is filled with theelectrolyte solution 508. The electrolyte solution described inEmbodiment 2 can be used for the electrolyte solution 508.

In the laminated secondary battery 500 illustrated in FIG. 13(A), thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for an electrical contactwith an external portion. For this reason, the positive electrodecurrent collector 501 and the negative electrode current collector 504may be arranged so as to be partly exposed to the outside of theexterior body 509. Alternatively, a lead electrode and the positiveelectrode current collector 501 or the negative electrode currentcollector 504 may be bonded to each other by ultrasonic welding, andinstead of the positive electrode current collector 501 and the negativeelectrode current collector 504, the lead electrode may be exposed tothe outside of the exterior body 509.

As the exterior body 509 of the laminated secondary battery 500, forexample, a laminate film having a three-layer structure can be employedin which a highly flexible metal thin film of aluminum, stainless steel,copper, nickel, or the like is provided over a film formed of a materialsuch as polyethylene, polypropylene, polycarbonate, ionomer, orpolyamide, and an insulating synthetic resin film of a polyamide-basedresin, a polyester-based resin, or the like is provided over the metalthin film as the outer surface of the exterior body.

FIG. 13(B) illustrates an example of a cross-sectional structure of thelaminated secondary battery 500. Although FIG. 13(A) illustrates anexample including only two current collectors for simplicity, an actualbattery includes a plurality of electrode layers.

The example in FIG. 13(B) includes 16 electrode layers. The secondarybattery 500 has flexibility even though including 16 electrode layers.FIG. 13(B) illustrates a structure including 8 layers of negativeelectrode current collectors 504 and 8 layers of positive electrodecurrent collectors 501, i.e., 16 layers in total. Note that FIG. 13(B)illustrates a cross section of the lead portion of the negativeelectrode, and the 8 negative electrode current collectors 504 arebonded to each other by ultrasonic welding. It is needless to say thatthe number of electrode layers is not limited to 16, and may be morethan 16 or less than 16. With a large number of electrode layers, thesecondary battery can have high capacity. With a small number ofelectrode layers, the secondary battery can have small thickness andhigh flexibility.

FIG. 14 and FIG. 15 each illustrate an example of the external view ofthe laminated secondary battery 500. In FIG. 14 and FIG. 15, thepositive electrode 503, the negative electrode 506, the separator 507,the exterior body 509, a positive electrode lead electrode 510, and anegative electrode lead electrode 511 are included.

FIG. 16(A) illustrates external views of the positive electrode 503 andthe negative electrode 506. The positive electrode 503 includes thepositive electrode current collector 501, and the positive electrodeactive material layer 502 is formed on a surface of the positiveelectrode current collector 501. The positive electrode 503 alsoincludes a region where the positive electrode current collector 501 ispartly exposed (hereinafter referred to as a tab region). The negativeelectrode 506 includes the negative electrode current collector 504, andthe negative electrode active material layer 505 is formed on a surfaceof the negative electrode current collector 504. The negative electrode506 also includes a region where the negative electrode currentcollector 504 is partly exposed, that is, a tab region. The areas andthe shapes of the tab regions included in the positive electrode and thenegative electrode are not limited to those illustrated in FIG. 16(A).

[Method for Manufacturing Laminated Secondary Battery]

Here, an example of a method for manufacturing the laminated secondarybattery whose external view is illustrated in FIG. 14 will be describedwith reference to FIG. 16(B) and FIG. 16(C).

First, the negative electrode 506, the separator 507, and the positiveelectrode 503 are stacked. FIG. 16(B) illustrates a stack including thenegative electrode 506, the separator 507, and the positive electrode503. An example described here includes 5 pairs of negative electrodesand 4 pairs of positive electrodes. Next, the tab regions of thepositive electrodes 503 are bonded to each other, and the positiveelectrode lead electrode 510 is bonded to the tab region of the positiveelectrode on the outermost surface. The bonding can be performed byultrasonic welding, for example. In a similar manner, the tab regions ofthe negative electrodes 506 are bonded to each other, and the negativeelectrode lead electrode 511 is bonded to the tab region of the negativeelectrode on the outermost surface.

After that, the negative electrode 506, the separator 507, and thepositive electrode 503 are placed over the exterior body 509.

Subsequently, the exterior body 509 is folded along a dashed line asillustrated in FIG. 16(C). Then, the outer edge of the exterior body 509is bonded. The bonding can be performed by thermocompression bonding,for example. At this time, a part (or one side) of the exterior body 509is left unbonded (to provide an inlet) so that the electrolyte solution508 can be introduced later.

Next, the electrolyte solution 508 is introduced into the exterior body509 from the inlet of the exterior body 509. The electrolyte solution508 is preferably introduced in a reduced pressure atmosphere or in aninert gas atmosphere. Lastly, the inlet is bonded. In the above manner,the laminated secondary battery 500 can be manufactured.

When the positive electrode active material particle 100 described inthe above embodiments is used in the positive electrode 503, thesecondary battery 500 with little deterioration and high safety can beobtained.

[Bendable Secondary Battery]

Next, an example of a bendable secondary battery is described withreference to FIG. 17 and FIG. 18.

FIG. 17(A) is a schematic top view of a bendable battery 250. FIG.17(B1), FIG. 17(B2), and FIG. 17(C) are schematic cross-sectional viewstaken along cutting line C1-C2, cutting line C3-C4, and cutting lineA1-A2, respectively, in FIG. 17(A). The battery 250 includes an exteriorbody 251, and a positive electrode 211 a and a negative electrode 211 bwhich are held in the exterior body 251. A lead 212 a electricallyconnected to the positive electrode 211 a and a lead 212 b electricallyconnected to the negative electrode 211 b are extended to the outside ofthe exterior body 251. In addition to the positive electrode 211 a andthe negative electrode 211 b, an electrolyte solution (not illustrated)is enclosed in a region surrounded by the exterior body 251.

The positive electrode 211 a and the negative electrode 211 b includedin the battery 250 are described with reference to FIG. 18. FIG. 18(A)is a perspective view illustrating the stacking order of the positiveelectrode 211 a, the negative electrode 211 b, and the separator 214.FIG. 18(B) is a perspective view illustrating the lead 212 a and thelead 212 b in addition to the positive electrode 211 a and the negativeelectrode 211 b.

As illustrated in FIG. 18(A), the battery 250 includes a plurality ofstrip-shaped positive electrodes 211 a, a plurality of strip-shapednegative electrodes 211 b, and a plurality of separators 214. Thepositive electrode 211 a and the negative electrode 211 b each include aprojected tab portion and a portion other than the tab. A positiveelectrode active material layer is formed on one surface of the positiveelectrode 211 a other than the tab portion, and a negative electrodeactive material layer is formed on one surface of the negative electrode211 b other than the tab portion.

The positive electrodes 211 a and the negative electrodes 211 b arestacked so that surfaces of the positive electrodes 211 a on each ofwhich the positive electrode active material layer is not formed are incontact with each other and that surfaces of the negative electrodes 211b on each of which the negative electrode active material layer is notformed are in contact with each other.

Furthermore, the separator 214 is provided between the surface of thepositive electrode 211 a on which the positive electrode active materiallayer is formed and the surface of the negative electrode 211 b on whichthe negative electrode active material layer is formed. In FIG. 18, theseparator 214 is shown by a dotted line for easy viewing.

In addition, as illustrated in FIG. 18(B), the plurality of positiveelectrodes 211 a are electrically connected to the lead 212 a in abonding portion 215 a. The plurality of negative electrodes 211 b areelectrically connected to the lead 212 b in a bonding portion 215 b.

Next, the exterior body 251 is described with reference to FIG. 17(B1),FIG. 17(B2), FIG. 17(C), and FIG. 17(D).

The exterior body 251 has a film-like shape and is folded in half withthe positive electrodes 211 a and the negative electrodes 211 b betweenfacing portions of the exterior body 251. The exterior body 251 includesa folded portion 261, a pair of seal portions 262, and a seal portion263. The pair of seal portions 262 is provided with the positiveelectrodes 211 a and the negative electrodes 211 b positionedtherebetween and thus can also be referred to as side seals. The sealportion 263 has portions overlapping with the lead 212 a and the lead212 b and can also be referred to as a top seal.

Part of the exterior body 251 that overlaps with the positive electrodes211 a and the negative electrodes 211 b preferably has a wave shape inwhich crest lines 271 and trough lines 272 are alternately arranged. Theseal portions 262 and the seal portion 263 of the exterior body 251 arepreferably flat.

FIG. 17(B1) shows a cross section cut along the part overlapping withthe crest line 271. FIG. 17(B2) shows a cross section cut along the partoverlapping with the trough line 272. FIG. 17(B1) and FIG. 17(B2)correspond to cross sections of the battery 250, the positive electrodes211 a, and the negative electrodes 211 b in the width direction.

Here, the distance between an end portion of the negative electrode 211b in the width direction, that is, the end portion of the negativeelectrode 211 b, and the seal portion 262 is referred to as a distanceLa. When the battery 250 changes in shape, for example, is bent, thepositive electrode 211 a and the negative electrode 211 b change inshape such that the positions thereof are shifted from each other in thelength direction as described later. At the time, if the distance La istoo short, the exterior body 251 and the positive electrode 211 a andthe negative electrode 211 b are rubbed hard against each other, so thatthe exterior body 251 is damaged in some cases. In particular, when ametal film of the exterior body 251 is exposed, there is concern thatthe metal film is corroded by the electrolyte solution. Thus, thedistance La is preferably set as long as possible. However, a too longdistance La increases the volume of the battery 250.

The distance La between the negative electrode 211 b and the sealportion 262 is preferably increased as the total thickness of thestacked positive electrodes 211 a and negative electrodes 211 b isincreased.

More specifically, when the total thickness of the stacked positiveelectrodes 211 a and negative electrodes 211 b is referred to as athickness t, the distance La is preferably 0.8 times or more and 3.0times or less, further preferably 0.9 times or more and 2.5 times orless, and still further preferably 1.0 times or more and 2.0 times orless as large as the thickness t. When the distance La is in this range,a compact battery which is highly reliable for bending can be obtained.

Furthermore, when the distance between the pair of seal portions 262 isreferred to as a distance Lb, it is preferable that the distance Lb besufficiently longer than the width of the positive electrode 211 a andthe negative electrode 211 b (here, a width Wb of the negative electrode211 b). In this case, even when the positive electrode 211 a and thenegative electrode 211 b come into contact with the exterior body 251 bychange in the shape of the battery 250 such as repeated bending, theposition of part of the positive electrode 211 a and the negativeelectrode 211 b can be shifted in the width direction; thus, thepositive and negative electrodes 211 a and 211 b and the exterior body251 can be effectively prevented from being rubbed against each other.

For example, the difference between the distance Lb between the pair ofseal portions 262 and the width Wb of the negative electrode 21 b ispreferably 1.6 times or more and 6.0 times or less, further preferably1.8 times or more and 5.0 times or less, and still further preferably2.0 times or more and 4.0 times or less as large as the total thicknesst of the positive electrode 211 a and the negative electrode 211 b.

In other words, the distance Lb, the width Wb, and the thickness tpreferably satisfy the relation of the following Formula 2.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{\frac{{Lb} - {Wb}}{2t} \geq a} & \left( {{Formula}\mspace{14mu} 2} \right)\end{matrix}$

In the formula, a is 0.8 or more and 3.0 or less, preferably 0.9 or moreand 2.5 or less, and further preferably 1.0 or more and 2.0 or less.

FIG. 17(C) illustrates a cross section including the lead 212 a andcorresponds to a cross section of the battery 250, the positiveelectrode 211 a, and the negative electrode 211 b in the lengthdirection. As illustrated in FIG. 17(C), in the folded portion 261, aspace 273 is preferably provided between end portions of the positiveelectrode 211 a and the negative electrode 211 b in the length directionand the exterior body 251.

FIG. 17(D) is a schematic cross-sectional view of the battery 250 thatis bent. FIG. 17(D) corresponds to a cross section along cutting lineB1-B2 in FIG. 17(A).

When the battery 250 is bent, a part of the exterior body 251 positionedon the outer side in bending is stretched and the other part positionedon the inner side changes in shape as it shrinks. More specifically, thepart of the exterior body 251 positioned on the outer side changes inshape such that the wave amplitude becomes smaller and the length of thewave period becomes larger. In contrast, the part of the exterior body251 positioned on the inner side changes in shape such that the waveamplitude becomes larger and the length of the wave period becomessmaller. When the exterior body 251 changes in shape in this manner,stress applied to the exterior body 251 due to bending is relieved, sothat a material itself that forms the exterior body 251 does not need toexpand and contract. As a result, the battery 250 can be bent with weakforce without damage to the exterior body 251.

Furthermore, as illustrated in FIG. 17(D), when the battery 250 is bent,the positions of the positive electrode 211 a and the negative electrode211 b are shifted relatively. At this time, ends of the stacked positiveelectrodes 211 a and negative electrodes 211 b on the seal portion 263side are fixed by a fixing member 217; thus, the plurality of positiveelectrodes 211 a and the plurality of negative electrodes 211 b are moreshifted at a position closer to the folded portion 261. Therefore,stress applied to the positive electrode 211 a and the negativeelectrode 211 b is relieved, and the positive electrode 211 a and thenegative electrode 211 b themselves do not need to expand and contract.As a result, the battery 250 can be bent without damage to the positiveelectrode 211 a and the negative electrode 211 b.

Furthermore, the space 273 provided between the positive electrode 211 aand the negative electrode 211 b and the exterior body 251 enables thepositive electrode 211 a and the negative electrode 211 b located on aninner side to be shifted relatively without being in contact with theexterior body 251 when the battery 250 is bent.

In the battery 250 illustrated in FIG. 17 and FIG. 18, the exteriorbody, the positive electrode 211 a, and the negative electrode 211 b areless likely to be damaged and the battery characteristics are lesslikely to deteriorate even when the battery 250 is repeatedly bent andunbent. When the positive electrode active material particle 100described in the above embodiments is used for the positive electrode211 a included in the battery 250, a battery with little deteriorationand high safety can be obtained.

Embodiment 4

In this embodiment, examples of electronic devices including thesecondary battery of one embodiment of the present invention aredescribed.

First, FIG. 19 shows examples of electronic devices including thebendable secondary battery described in Embodiment 3. Examples of anelectronic device including a bendable secondary battery includetelevision sets (also referred to as televisions or televisionreceivers), monitors of computers or the like, digital cameras, digitalvideo cameras, digital photo frames, mobile phones (also referred to ascellular phones or mobile phone devices), portable game machines,portable information terminals, audio reproducing devices, and largegame machines such as pachinko machines.

In addition, a flexible secondary battery can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 19(A) illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a secondary battery 7407.

FIG. 19(B) illustrates the mobile phone 7400 that is bent. When thewhole mobile phone 7400 is curved by external force, the secondarybattery 7407 included in the mobile phone 7400 is also curved. FIG.19(C) illustrates the curved secondary battery 7407. The secondarybattery 7407 is a thin secondary battery. The secondary battery 7407 iscurved and fixed. Note that the secondary battery 7407 includes a leadelectrode electrically connected to a current collector.

FIG. 19(D) illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a secondary battery 7104. FIG. 19(E)illustrates the bent secondary battery 7104. When the curved secondarybattery 7104 is on a user's arm, the housing changes its form and thecurvature of a part or the whole of the secondary battery 7104 ischanged. Note that the radius of curvature of a curve at a point refersto the radius of the circular arc that best approximates the curve atthat point, and the reciprocal of the radius of curvature is referred toas a curvature. Specifically, part or the whole of the housing or themain surface of the secondary battery 7104 is changed in the range ofradius of curvature from 40 mm to 150 mm. When the radius of curvatureat the main surface of the secondary battery 7104 is greater than orequal to 40 mm and less than or equal to 150 mm, the reliability can bekept high.

FIG. 19(F) illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by setting the operation systemincorporated in the portable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. For example, mutual communication between theportable information terminal 7200 and a headset capable of wirelesscommunication can be performed, and thus hands-free calling is possible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200includes the secondary battery of one embodiment of the presentinvention. For example, the secondary battery 7104 illustrated in FIG.19(E) can be provided in the housing 7201 while being curved, or can beprovided in the band 7203 such that it can be curved.

The portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example, a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor, a touch sensor, apressure sensitive sensor, an acceleration sensor, or the like ispreferably mounted.

FIG. 19(G) illustrates an example of an armband display device. Adisplay device 7300 includes a display portion 7304 and the secondarybattery of one embodiment of the present invention. The display device7300 can include a touch sensor in the display portion 7304 and canserve as a portable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

Next, FIG. 20(A) and FIG. 20(B) illustrate an example of a foldabletablet terminal. A tablet terminal 9600 illustrated in FIG. 20(A) andFIG. 20(B) includes a housing 9630 a, a housing 9630 b, a movableportion 9640 connecting the housing 9630 a and the housing 9630 b, adisplay portion 9631, a display mode changing switch 9626, a powerswitch 9627, a power saving mode changing switch 9625, a fastener 9629,and an operation switch 9628. A flexible panel is used for the displayportion 9631, whereby a tablet terminal with a larger display portioncan be provided. FIG. 20(A) illustrates the tablet terminal 9600 that isopened, and FIG. 20(B) illustrates the tablet terminal 9600 that isclosed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousing 9630 a and the housing 9630 b. The power storage unit 9635 isprovided across the housing 9630 a and the housing 9630 b, passingthrough the movable portion 9640.

Part of the display portion 9631 can be a touch panel region and datacan be input when a displayed operation key is touched. A switchingbutton for showing/hiding a keyboard of the touch panel is touched witha finger, a stylus, or the like, so that keyboard buttons can bedisplayed on the display portion 9631.

The display mode switch 9626 can switch the display between a portraitmode and a landscape mode, and between monochrome display and colordisplay, for example. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. Anotherdetection device including a sensor for detecting inclination, such as agyroscope sensor or an acceleration sensor, may be incorporated in thetablet terminal, in addition to the optical sensor.

The tablet terminal is closed in FIG. 20(B). The tablet terminalincludes the housing 9630, a solar cell 9633, and a charge and dischargecontrol circuit 9634 including a DC-DC converter 9636. The secondarybattery of one embodiment of the present invention is used as the powerstorage unit 9635.

The tablet terminal 9600 can be folded such that the housing 9630 a andthe housing 9630 b overlap with each other when not in use. Thus, thedisplay portion 9631 can be protected, which increases the durability ofthe tablet terminal 9600. Since the power storage unit 9635 using thesecondary battery of one embodiment of the present invention has highcapacity and excellent cycle characteristics, the tablet terminal whichcan be used for a long time for a long period can be provided.

The tablet terminal illustrated in FIG. 20(A) and FIG. 20(B) can alsohave a function of displaying various kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, or the time on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thepower storage unit 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 20(B) will be described with reference to ablock diagram in FIG. 20(C). The solar cell 9633, the power storage unit9635, the DC-DC converter 9636, a converter 9637, switches SW1 to SW3,and the display portion 9631 are illustrated in FIG. 20(C), and thepower storage unit 9635, the DC-DC converter 9636, the converter 9637,and the switches SW1 to SW3 correspond to the charge and dischargecontrol circuit 9634 in FIG. 20(B).

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDC-DC converter 9636 to a voltage for charging the power storage unit9635. When the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to a voltageneeded for operating the display portion 9631. When display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on, so that the power storage unit 9635 can becharged.

Note that the solar cell 9633 is described as an example of a powergeneration means: however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module that transmits and receives power wirelessly(without contact) to charge the battery or with a combination of othercharging means.

FIG. 21 illustrates other examples of electronic devices. In FIG. 21, adisplay device 8000 is an example of an electronic device using asecondary battery 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, the secondary battery 8004, and thelike. The secondary battery 8004 of one embodiment of the presentinvention is provided in the housing 8001. The display device 8000 canreceive electric power from a commercial power supply, or use electricpower stored in the secondary battery 8004. Thus, the display device8000 can operate with the use of the secondary battery 8004 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 21, an installation lighting device 8100 is an example of anelectronic device using a secondary battery 8103 of one embodiment ofthe present invention. Specifically, the lighting device 8100 includes ahousing 8101, a light source 8102, the secondary battery 8103, and thelike. Although FIG. 21 illustrates the case where the secondary battery8103 is provided in a ceiling 8104 on which the housing 8101 and thelight source 8102 are installed, the secondary battery 8103 may beprovided in the housing 8101. The lighting device 8100 can receiveelectric power from a commercial power supply, or use electric powerstored in the secondary battery 8103. Thus, the lighting device 8100 canoperate with the use of the secondary battery 8103 of one embodiment ofthe present invention as an uninterruptible power supply even whenelectric power cannot be supplied from a commercial power supply due topower failure or the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 21 as an example, the secondarybattery of one embodiment of the present invention can be used as aninstallation lighting device provided in, for example, a sidewall 8105,a floor 8106, a window 8107, or the like other than the ceiling 8104, orcan be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and a light-emittingelement such as an LED or an organic EL element are given as examples ofthe artificial light source.

In FIG. 21, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including asecondary battery 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the secondary battery 8203, and the like. Although FIG. 21illustrates the case where the secondary battery 8203 is provided in theindoor unit 8200, the secondary battery 8203 may be provided in theoutdoor unit 8204. Alternatively, the secondary batteries 8203 may beprovided in both the indoor unit 8200 and the outdoor unit 8204. The airconditioner can receive electric power from a commercial power supply,or use electric power stored in the secondary battery 8203. Particularlyin the case where the secondary batteries 8203 are provided in both theindoor unit 8200 and the outdoor unit 8204, the air conditioner canoperate with the use of the secondary battery 8203 of one embodiment ofthe present invention as an uninterruptible power supply even whenelectric power cannot be supplied from a commercial power supply due topower failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 21 as an example, thesecondary battery of one embodiment of the present invention can be usedin an air conditioner in which the functions of an indoor unit and anoutdoor unit are integrated in one housing.

In FIG. 21, an electric refrigerator-freezer 8300 is an example of anelectronic device using a secondary battery 8304 of one embodiment ofthe present invention. Specifically, the electric refrigerator-freezer8300 includes a housing 8301, a refrigerator door 8302, a freezer door8303, the secondary battery 8304, and the like. The secondary battery8304 is provided in the housing 8301 in FIG. 21. The electricrefrigerator-freezer 8300 can receive electric power from a commercialpower supply, or use electric power stored in the secondary battery8304. Thus, the electric refrigerator-freezer 8300 can operate with theuse of the secondary battery 8304 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

In addition, power can be stored in the secondary battery in a timeperiod when electronic devices are not used, particularly when theproportion of the amount of power which is actually used to the totalamount of power which can be supplied from a commercial power source(such a proportion referred to as a usage rate of power) is low, wherebyan increase in the usage rate of power can be reduced in a time periodwhen the electronic devices are used. For example, in the case of theelectric refrigerator-freezer 8300, power is stored in the secondarybattery 8304 in night time when the temperature is low and therefrigerator door 8302 and the freezer door 8303 are not opened andclosed. On the other hand, in daytime when the temperature is high andthe refrigerator door 8302 and the freezer door 8303 are opened andclosed, the secondary battery 8304 is used as an auxiliary power source;thus, the usage rate of power in daytime can be reduced.

The secondary battery of one embodiment of the present invention can beused in a variety of electronic devices as well as the above electronicdevices. According to one embodiment of the present invention, thesecondary battery can have little deterioration and high safety. Thus,when the secondary battery of one embodiment of the present invention isused in the electronic devices described in this embodiment, electronicdevices with longer lifetime and higher safety can be obtained. Thisembodiment can be implemented in appropriate combination with the otherembodiments.

Embodiment 5

In this embodiment, examples of vehicles including the secondary batteryof one embodiment of the present invention will be described.

The use of secondary batteries in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIG. 22 illustrates examples of a vehicle using the secondary battery ofone embodiment of the present invention. An automobile 8400 illustratedin FIG. 22(A) is an electric vehicle that runs on the power of anelectric motor. Alternatively, the automobile 8400 is a hybrid electricvehicle capable of driving appropriately using either an electric motoror an engine. The use of a secondary battery of one embodiment of thepresent invention can provide a high-mileage vehicle. The automobile8400 includes the secondary battery. The secondary battery is used notonly for driving an electric motor 8406, but also for supplying electricpower to a light-emitting device such as a headlight 8401 or a roomlight (not illustrated).

The secondary battery can also supply electric power to a display deviceof a speedometer, a tachometer, or the like included in the automobile8400. Furthermore, the secondary battery can supply electric power to asemiconductor device included in the automobile 8400, such as anavigation system.

An automobile 8500 illustrated in FIG. 22(B) can be charged when asecondary battery 8024 included in the automobile 8500 is supplied withelectric power through external charging equipment by a plug-in system,a contactless power feeding system or the like. In FIG. 22(B), thesecondary battery 8024 mounted on the automobile 8500 is charged withthe use of a ground-based charging apparatus 8021 through a cable 8022.In charging, a given method such as CHAdeMO (registered trademark) orCombined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. With the use of a plug-in technique, thesecondary battery 8024 mounted on the automobile 8500 can be charged bybeing supplied with electric power from the outside, for example. Thecharging can be performed by converting AC electric power into DCelectric power through a converter such as an AC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the vehicle stops but also whenmoves. In addition, the contactless power feeding system may be utilizedto perform transmission and reception of electric power betweenvehicles. A solar cell may be provided in the exterior of the vehicle tocharge the secondary battery when the vehicle stops or moves. To supplyelectric power in such a contactless manner, an electromagneticinduction method or a magnetic resonance method can be used.

FIG. 22(C) shows an example of a motorcycle using the secondary batteryof one embodiment of the present invention. A motor scooter 8600illustrated in FIG. 22(C) includes a secondary battery 8602, sidemirrors 8601, and indicators 8603. The secondary battery 8602 can supplyelectric power to the indicators 8603.

Furthermore, in the motor scooter 8600 illustrated in FIG. 22(C), thesecondary battery 8602 can be held in a storage unit under seat 8604.The secondary battery 8602 can be held in the storage unit under seat8604 even with a small size.

According to one embodiment of the present invention, the secondarybattery can have little deterioration and high safety. Thus, when thesecondary battery is mounted on a vehicle, a reduction in mileage,acceleration performance, or the like can be inhibited. In addition, ahighly safe vehicle can be achieved. Furthermore, the secondary batterymounted on the vehicle can be used as a power source for supplyingelectric power to products other than the vehicle. In such a case, theuse of a commercial power source can be avoided at peak time of electricpower demand, for example. If the use of a commercial power source canbe avoided at peak time of electric power demand, the avoidance cancontribute to energy saving and a reduction in carbon dioxide emissions.Moreover, the secondary battery with little deterioration and highsafety can be used for a long period; thus, the use amount of raremetals such as cobalt can be reduced.

This embodiment can be implemented in appropriate combination with theother embodiments.

Example 1

In this example, a positive electrode active material particle includingmagnesium, fluorine, and oxygen in a crystal grain boundary and theperiphery thereof was fabricated and the concentration distribution in acrystal grain and a crystal grain boundary in the active material wasfound by TEM observation and STEM-EDX analysis. Sample A was prepared asa sample of one embodiment of the present invention. As Sample A,lithium nickel-manganese-cobalt oxide including magnesium, fluorine, andoxygen in a crystal grain boundary and the periphery thereof wasfabricated. Lithium nickel-manganese-cobalt oxide was assumed to have acomposition of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂.LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ has a layered rock-salt crystal structure.

<Fabrication of Sample A>

The fabrication of Sample A will be described.

Starting materials were prepared as shown in Step S11 in the flowchartof FIG. 3. Lithium carbonate (Li₂CO₃) as a lithium source, nickel oxide(NiO) as a nickel source, manganese dioxide (MnO₂) as a manganesesource, tricobalt tetroxide (Co₃O₄) as a cobalt source, magnesium oxide(MgO) as a magnesium source, and lithium fluoride (LiF) as a fluorinesource were weighed. Specifically, 3.1398 g (42.49 mmol) of Li₂CO₃,2.1159 g (28.33 mmol) of NiO, 2.4627 g (28.33 mmol) of MnO₂, 2.2033 g(9.15 mmol) of Co₃O₄, 0.0343 g (0.85 mmol) of MgO, and 0.0441 g (1.70mmol) of LiF were weighed. According to this, the ratio m of the numberof magnesium atoms to the total number of atoms of nickel, manganese,and cobalt is 0.010 (1.0%). In addition, the ratio n of the number offluorine atoms to the number of magnesium atoms is 2.0. Note that Li₂CO₃used is a product of Kojundo Chemical Laboratory Co., Ltd. (catalog No.LIH06XB). NiO used is a product of Kojundo Chemical Laboratory Co., Ltd.(catalog No. NIO04PB). MnO₂ used is a product of Kojundo ChemicalLaboratory Co., Ltd. (catalog No. MNO03PB). Co₃O₄ used is a product ofKojundo Chemical Laboratory Co., Ltd. (catalog No. COO09PB). MgO used isa product of Kojundo Chemical Laboratory Co., Ltd. (catalog No.MGO12PB). LiF used is a product of Kojundo Chemical Laboratory Co., Ltd.(catalog No. LIH10XB).

Next, as shown in Step S12, the starting materials weighed in Step S11were mixed. A wet ball mill was used for the mixing. Specifically, withuse of a 3-mmφ-ball and acetone as a solvent, grinding and mixing wereperformed at a spinning rate of 300 rpm for 2 hours.

Next, as shown in Step S13, a first heating was performed on thematerials mixed in Step S12. In the first heating, with use of a mufflefurnace, the temperature was increased from room temperature to 1000° C.at a temperature rising rate of 200° C./h and heating at 1000° C. wascontinued for 10 hours. The heating was performed in a dry airatmosphere with a flow rate of 10 L/min.

Through the first heating in Step S13, lithium nickel-manganese-cobaltoxide can be synthesized. Note that part of the magnesium and fluorineat this stage probably forms a solid solution in the crystal grainboundary and the crystal grain.

Next, as shown in Step S14, the materials heated in Step S13 were cooledto room temperature to obtain a synthetic material 1. After the cooling,the synthetic material 1 was subjected to crushing treatment, wherebythe particle size of the synthetic material 1 was reduced. A 53-μm meshwas used for the crushing treatment.

Next, as shown in Step S15, a second heating was performed on thesynthetic material 1 obtained in Step S14. In the second heating, withuse of a muffle furnace, the temperature was increased from roomtemperature to 800° C. at a temperature rising rate of 200° C./h andheating at 800° C. was continued for 2 hours. The heating was performedin a dry air atmosphere with a flow rate of 10 L/min.

The second heating in Step S15 promotes segregation of the magnesium andfluorine contained in the starting materials into the crystal grainboundary of lithium nickel-manganese-cobalt oxide.

Next, as shown in Step S16, the synthetic material 1 heated in Step S15was cooled to room temperature and collected, so that Sample A wasobtained.

<TEM Observation, STEM Observation, and EDX Measurement>

Then, Sample A was thinned by focused ion beam (FIB) and a cross sectionof Sample A was observed with TEM and STEM. Furthermore, the compositionanalysis of the cross section of Sample A was performed by EDXmeasurement. The TEM and STEM observation and the EDX measurement wereperformed with JEM-ARM200F manufactured by JEOL Ltd., at an accelerationvoltage of 200 kV and a beam diameter of approximately 0.1 nmφ.

In the EDX measurement, an energy dispersive X-ray spectrometerJED-2300T manufactured by JEOL Ltd. was used as an elementary analysisapparatus, and a Si drift detector was used to detect an X-ray. Thelower detection limit of the EDX plane analysis was approximately 1atomic %. Note that the EDX measurement allows detection of elementsfrom boron (B), atomic number 5, to uranium (U), atomic number 92.

FIG. 23(A) shows a cross-sectional TEM image (a bright-field image) ofSample A. The magnification of FIG. 23(A) is 100,000 times. In FIG.23(A), a region where the concentration (luminance) of the TEM image issubstantially uniform probably has a substantially uniform crystalorientation, i.e., a single crystal. A region where the concentration(luminance) of the TEM image changes is probably a grain boundary. FIG.23(B) shows a schematic diagram corresponding to FIG. 23(A). As shown inFIG. 23(A) and FIG. 23(B), the positive electrode active materialparticle was found to include a crystal grain boundary 1103 between aplurality of crystal grains 1101 and a crystal grain.

FIG. 24(A) shows a cross-sectional STEM image (a bright-field image) ofSample A, and FIG. 24(B) shows a HAADF-STEM image of the same point. Themagnification of FIG. 24(A) and FIG. 24(B) is 8,000,000 times. A crystallattice image was observed in a crystal grain region in FIG. 24(A) andFIG. 24(B).

Next, EDX spectra of a cross section of Sample A will be described. Inthe EDX measurement, measurement points were subjected to electron beamirradiation and the energy of characteristic X-ray generated by theirradiation and its frequency were measured, whereby the EDX spectrawere obtained. FIG. 25 shows a cross-sectional HAADF-STEM image ofSample A and the EDX measurement points. The EDX measurement pointsconsist of five points, point 1 to point 5. The point 2 to the point 4are in the crystal grain boundary and the periphery thereof, and thepoint 1 and the point 5 are in a position apart from the crystal grainboundary, i.e., in a crystal grain. FIG. 26 shows the EDX spectra andthe quantification results of the point 1; FIG. 27, the point 2; FIG.28, the point 3; FIG. 29, the point 4; and FIG. 30, the point 5. In FIG.26 to FIG. 30, the horizontal axis represents the energy ofcharacteristic X-ray [keV] and the vertical axis represents thecharacteristic X-ray intensity [Counts].

The peaks observed at the point 1 to the point 5 are derived fromelectron transition to the K shell in carbon (C), oxygen (O), fluorine(F), magnesium (Mg), silicon (Si), phosphorus (P), sulfur (S), calcium(Ca), manganese (Mn), cobalt (Co), and nickel (Ni). The obtained spectrawere separated into those of the respective elements, so that the atomicconcentrations were obtained.

Next, the EDX plane analysis will be described. The measurement in whicha region is measured while scanning and evaluated two-dimensionally isreferred to as surface analysis in some cases. In this example, the EDXmeasurement was performed on 256×256 points in the region.

FIG. 31(A) shows a HAADF-STEM image of the region of Sample A that wassubjected to the EDX plane analysis. The EDX plane analysis wasperformed in a region including a crystal grain and a crystal grainboundary. FIG. 31(B) shows a mapping image of carbon in the EDX planeanalysis of the region illustrated in FIG. 31(A); FIG. 31(C), oxygen;FIG. 31(D), fluorine; FIG. 31(E), magnesium; FIG. 31(F), silicon; FIG.32(A), phosphorus; FIG. 32(B), sulfur; FIG. 32(C), calcium; FIG. 32(D),manganese; FIG. 32(E), cobalt; and FIG. 32(F), nickel.

FIG. 31(B) to FIG. 31(F) and FIG. 32(A) to FIG. 32(F) each show theintensity mapping of characteristic X-ray obtained by the EDXmeasurement; a measurement point with a low characteristic X-rayintensity is denoted with a pale color (white), and a measurement pointwith a higher characteristic X-ray intensity is denoted with a darkercolor (black). In other words, the pale color (white) measurement pointmeans a low atomic concentration whereas the dark color (black)measurement point means a high atomic concentration. Note that in FIG.31(B) to FIG. 31(F) and FIG. 32(A) to FIG. 32(F), the scale of thecharacteristic X-ray intensity differs for each element so as to clearlyshow the distribution in the region.

As shown in FIG. 31(B) to FIG. 31(F) and FIG. 32(A) to FIG. 32(F), theconcentrations of fluorine, magnesium, silicon, and calcium were foundto be high in the crystal grain boundary and the periphery thereof. Notethat silicon and calcium were probably contained in a reagent used as araw material.

Data in linear regions was extracted from the EDX plane analysis shownin FIG. 31(B) to FIG. 31(F) and FIG. 32(A) to FIG. 32(F), and thedistribution of the atomic concentrations in the positive electrodeactive material particle was evaluated. Such one-dimensional evaluationof the linear region is referred to as a linear analysis in some cases.

FIG. 33(A) shows a HAADF-STEM image of the region of Sample A that wassubjected to the EDX linear analysis. In FIG. 33(A), the regionsubjected to the EDX linear analysis is denoted by an arrow. The EDXlinear analysis was performed on a crystal grain, a crystal grainboundary, and a region across the crystal grain.

FIG. 34(A) shows the atomic concentration of carbon in the EDX linearanalysis of the region illustrated in FIG. 33(A); FIG. 34(B), oxygen;FIG. 34(C), fluorine; FIG. 34(D), magnesium; FIG. 34(E), silicon; FIG.34(F), phosphorus; FIG. 35(A), sulfur; FIG. 35(B), calcium; FIG. 35(C),manganese; FIG. 35(D), cobalt; and FIG. 35(E), nickel.

In FIG. 34(A) to FIG. 34(F) and FIG. 35(A) to FIG. 35(E), the horizontalaxis represents the distance [nm] and the vertical axis represents theatomic concentration [atomic %]. The distance on the horizontal axis isshown so as to increase from the starting point (distance=0 nm), whichis indicated as a black dot on one end of the arrow illustrated in FIG.34(A), to the other end (ending portion). The atomic concentration onthe vertical axis shows the percentage of the number of atoms for eachelement with respect to the total number of atoms of carbon, oxygen,fluorine, magnesium, silicon, phosphorus, sulfur, calcium, manganese,cobalt, and nickel as 100 atomic %.

As shown in FIG. 33(A), FIG. 34(A) to FIG. 34(F), and FIG. 35(A) to FIG.35(E), the concentrations of fluorine, magnesium, silicon, and calciumwere found to be higher in the crystal grain boundary and the peripherythereof than in the crystal grain region. It was also found that thecrystal grain boundary and the periphery thereof had a region with awidth greater than or equal to 1 nm and less than or equal to 10 nm.

The crystal grain boundary and the periphery thereof were found toinclude oxygen, magnesium, and fluorine. The crystal grain boundary andthe periphery thereof were found to include magnesium oxide. Fluorine isprobably substituted for part of oxygen included in magnesium oxide.

In contrast, fluorine, magnesium, silicon, and calcium were at the levelof the lower detection limit in the crystal grain region.

Phosphorus and sulfur were at the level of the lower detection limit inthe crystal grain and the crystal grain boundary.

The carbon concentration detected in the crystal grain and the crystalgrain boundary probably includes carbon derived from a carbon coat filmused as a protective film. It was thus not possible to determine theactual carbon concentration in the crystal grain and the crystal grainboundary.

The atomic concentrations of manganese, cobalt, and nickel, which aretransition metals, were found to be lower in the crystal grain boundaryand the periphery thereof than in the crystal grain.

FIG. 35(F) shows the total atomic concentration of nickel, manganese,and cobalt, which are transition metals. In FIG. 35(F), the horizontalaxis represents the distance [nm] and the vertical axis represents thetotal atomic concentration [atomic %] of nickel, manganese, and cobalt(Ni+Mn+Co). Specifically, the total atomic concentration of nickel,manganese, and cobalt (Ni+Mn+Co) is the sum of atomic concentrations ofnickel, manganese, and cobalt in each measurement point of EDX. Thetotal atomic concentration of nickel, manganese, and cobalt (Ni+Mn+Co)in Sample A can be regarded as the atomic concentration of thetransition metal. As shown in FIG. 35(F), the atomic concentration ofthe transition metal was found to be prone to be lower in the crystalgrain boundary and the periphery thereof than that in the crystal grainregion. It was also found that the atomic concentration of thetransition metal in the crystal grain region was substantially uniformwithout large variation.

FIG. 36(A) shows the ratio of the atomic concentration of magnesium (Mg)to the atomic concentration of the transition metal in the crystalgrain. In FIG. 36(A), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the atomic concentrationof magnesium to the atomic concentration of the transition metal in thecrystal grain (Mg/Tr-Metal) (arb, unit).

The atomic concentration of the transition metal (Tr-Metal) in thecrystal grain is described. The average atomic concentration of thetransition metals in the crystal grain was used as the atomicconcentration of the transition metal (Tr-Metal) in the crystal grain.Specifically, the crystal grain region was defined as a region having amagnesium (Mg) atomic concentration at a lower detection limit, and theaverage atomic concentration of the transition metals in that region wascalculated. The crystal grain region used for the calculation of theaverage value is indicated by arrows in FIG. 35(F).

As shown in FIG. 36(A), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the atomic concentration of the transition metal(Mg/Tr-Metal) in the crystal grain is greater than or equal to 0.030.Magnesium was found to be segregated in the crystal grain boundary andthe periphery thereof. The crystal grain boundary and the peripherythereof probably include magnesium oxide. Sample A of one embodiment ofthe present invention includes magnesium oxide in the crystal grainboundary and the periphery thereof, offering chemical and structuralstability to the positive electrode active material particle, so thatdeterioration of the positive electrode active material, such asdissolution of the transition metal to an electrolyte solution, releaseof oxygen, and unstable crystal structure, can be inhibited. Inaddition, cracking of the positive electrode active material particlecan be inhibited. Release of oxygen from the positive electrode activematerial particle can also be inhibited. The use of such a positiveelectrode active material particle can inhibit deterioration of a powerstorage device. In addition, a highly safe power storage device can beachieved. When the charge voltage is increased, the amount of lithiumincluded in a positive electrode is reduced when charging, and thecrystal structure of a positive electrode active material particle isprone to change; thus, Sample A is particularly preferable as thepositive electrode active material particle.

FIG. 36(B) shows the ratio of the fluorine atomic concentration to theatomic concentration of the transition metal (Tr-Metal) in the crystalgrain. In FIG. 36(B), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the fluorine atomicconcentration to the atomic concentration of the transition metal in thecrystal grain (F/Tr-Metal).

As shown in FIG. 36(B), the crystal grain boundary and the periphery,thereof were found to include a region where the ratio of the fluorineatomic concentration to the atomic concentration of the transition metal(F/Tr-Metal) in the crystal grain is greater than or equal to 0.030.Fluorine in the crystal grain boundary and the periphery thereof wasfound to contribute to efficient segregation of magnesium in the crystalgrain boundary and the periphery thereof.

Note that in this specification and the like. “the ratio of the atomicconcentration” is synonymous with “the ratio of the number of atoms”,and “the ratio of the atomic concentration” can be replaced with “theratio of the number of atoms”. That is, the value of Mg/Tr-metal can beregarded as the ratio of the magnesium atomic concentration to theatomic concentration of the transition metal in the crystal grain, andcan also be regarded as the ratio of the number of magnesium atoms tothe number of atoms of the transition metals in the crystal grain.

FIG. 36(C) shows the ratio of the magnesium (Mg) atomic concentration tothe total atomic concentration of nickel, manganese, and cobalt(Ni+Mn+Co) at each measurement point of EDX. In FIG. 36(C), thehorizontal axis represents the distance [nm] and the vertical axisrepresents the ratio of the magnesium atomic concentration to the totalatomic concentration of nickel, manganese, and cobalt (Mg/(Ni+Mn+Co)) ateach measurement point of EDX.

The total atomic concentration of nickel, manganese, and cobalt(Ni+Mn+Co) at each measurement point of EDX is the same as that in thedata shown in FIG. 35(F).

As shown in FIG. 36(C), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the total atomic concentration of nickel,manganese, and cobalt (Mg/(Ni+Mn+Co)) in the crystal grain is greaterthan or equal to 0.030. Magnesium was found to be segregated in thecrystal grain boundary and the periphery thereof.

FIG. 36(D) shows the ratio of the fluorine atomic concentration to thetotal atomic concentration of nickel, manganese, and cobalt (Ni+Mn+Co)at each measurement point of EDX. In FIG. 36(D), the horizontal axisrepresents the distance [nm] and the vertical axis represents the ratioof the fluorine atomic concentration to the total atomic concentrationof nickel, manganese, and cobalt (F/(Ni+Mn+Co)) at each measurementpoint of EDX.

As shown in FIG. 36(D), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the fluorineatomic concentration to the atomic concentration of the transition metal(F/(Ni+Mn+Co)) in the crystal grain is greater than or equal to 0.030.Fluorine in the crystal grain boundary and the periphery thereof wasfound to contribute to efficient segregation of magnesium in the crystalgrain boundary and the periphery thereof.

Similar EDX measurement was performed on another portion of Sample A.

FIG. 37(A) shows a HAADF-STEM image of the region of Sample A that wassubjected to the EDX plane analysis. The EDX plane analysis wasperformed in a region including a crystal grain and a crystal grainboundary. FIG. 37(B) shows a mapping image of carbon in the EDX planeanalysis of the region illustrated in FIG. 37(A); FIG. 37(C), oxygen;FIG. 37(D), fluorine; FIG. 37(E), magnesium; FIG. 37(F), silicon; FIG.38(A), phosphorus; FIG. 38(B), sulfur; FIG. 38(C), calcium; FIG. 38(D),manganese; FIG. 38(E), cobalt; and FIG. 38(F), nickel.

FIG. 37(B) to FIG. 37(F) and FIG. 38(A) to FIG. 38(F) each show theintensity mapping of characteristic X-ray obtained by the EDXmeasurement; a measurement point with a low characteristic X-rayintensity is denoted with a pale color (white), and a measurement pointwith a higher characteristic X-ray intensity is denoted with a darkercolor (black). In other words, the pale color (white) measurement pointmeans a low atomic concentration whereas the dark color (black)measurement point means a high atomic concentration. Note that in FIG.37(B) to FIG. 37(F) and FIG. 38(A) to FIG. 38(F), the scale of thecharacteristic X-ray intensity differs for each element so as to clearlyshow the distribution in the region.

As shown in FIG. 37(B) to FIG. 37(F) and FIG. 38(A) to FIG. 38(F), theconcentrations of fluorine, magnesium, silicon, and calcium were foundto be high in the crystal grain boundary and the periphery thereof. Notethat silicon and calcium were probably contained in a reagent used as araw material.

Data in linear regions was extracted from the EDX plane analysis shownin FIG. 37(B) to FIG. 37(F) and FIG. 38(A) to FIG. 38(F), and thedistribution of the atomic concentrations in the positive electrodeactive material particle was evaluated.

FIG. 33(B) shows a HAADF-STEM image of the region of Sample A that wassubjected to the EDX linear analysis. In FIG. 33(B), the regionsubjected to the EDX linear analysis is denoted by an arrow. The EDXlinear analysis was performed on a crystal grain, a crystal grainboundary, and a region across the crystal grain.

FIG. 39(A) shows the atomic concentration of carbon in the EDX linearanalysis of the region illustrated in FIG. 33(B); FIG. 39(B), oxygen;FIG. 39(C), fluorine; FIG. 39(D), magnesium; FIG. 39(E), silicon; FIG.39(F), phosphorus; FIG. 40(A), sulfur; FIG. 40(B), calcium; FIG. 40(C),manganese; FIG. 40(D), cobalt; and FIG. 40(E), nickel.

In FIG. 39(A) to FIG. 39(F) and FIG. 40(A) to FIG. 40(E), the horizontalaxis represents the distance [nm] and the vertical axis represents theatomic concentration [atomic %]. The distance on the horizontal axis isshown so as to increase from the starting point (distance=0 nm), whichis indicated as a black dot on one end of the arrow illustrated in FIG.33(B), to the other end (ending portion). The atomic concentration onthe vertical axis shows the percentage of the number of atoms for eachelement with respect to the total number of atoms of carbon, oxygen,fluorine, magnesium, silicon, phosphorus, sulfur, calcium, manganese,cobalt, and nickel as 100 atomic %.

As shown in FIG. 33(B), FIG. 39(A) to FIG. 39(F), and FIG. 40(A) to FIG.40(E), the concentrations of fluorine, magnesium, silicon, and calciumwere found to be higher in the crystal grain boundary and the peripherythereof than in the crystal grain region. It was also found that thecrystal grain boundary and the periphery thereof had a region with awidth greater than or equal to 1 nm and less than or equal to 10 nm.

The crystal grain boundary and the periphery thereof were found toinclude oxygen, magnesium, and fluorine. The crystal grain boundary andthe periphery thereof were found to include magnesium oxide. Fluorine isprobably substituted for part of oxygen included in magnesium oxide.

In contrast, fluorine, magnesium, silicon, and calcium were at the levelof the lower detection limit in the crystal grain region.

Phosphorus and sulfur were at the level of the lower detection limit inthe crystal grain and the crystal grain boundary.

The carbon concentration detected in the crystal grain and the crystalgrain boundary probably includes carbon derived from a carbon coat filmused as a protective film. It was thus not possible to determine theactual carbon concentration in the crystal grain and the crystal grainboundary.

The atomic concentrations of manganese, cobalt, and nickel, which aretransition metals, were found to be lower in the crystal grain boundaryand the periphery thereof than in the crystal grain.

FIG. 40(F) shows the total atomic concentration of nickel, manganese,and cobalt, which are transition metals. In FIG. 40(F), the horizontalaxis represents the distance [nm] and the vertical axis represents thetotal atomic concentration [atomic %] of nickel, manganese, and cobalt(Ni+Mn+Co). The total atomic concentration of nickel, manganese, andcobalt (Ni+Mn+Co) in Sample A can be regarded as the atomicconcentration of the transition metal. As shown in FIG. 40(F), theatomic concentration of the transition metal was found to be prone to belower in the crystal grain boundary and the periphery thereof than thatin the crystal grain region. It was also found that the atomicconcentration of the transition metal in the crystal grain region wassubstantially uniform without large variation.

FIG. 41(A) shows the ratio of the atomic concentration of magnesium (Mg)to the atomic concentration of the transition metal in the crystalgrain. In FIG. 41(A), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the atomic concentrationof magnesium to the atomic concentration of the transition metal in thecrystal grain (Mg/Tr-Metal).

The average atomic concentration of the transition metals in the crystalgrain was used as the atomic concentration of the transition metal(Tr-Metal) in the crystal grain. The crystal grain region used for thecalculation of the average value is indicated by arrows in FIG. 40(F).

As shown in FIG. 41(A), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the atomic concentration of the transition metal(Mg-Tr-Metal) in the crystal grain is greater than or equal to 0.030.Magnesium was found to be segregated in the crystal grain boundary andthe periphery thereof. The crystal grain boundary and the peripherythereof probably include magnesium oxide. Sample A of one embodiment ofthe present invention includes magnesium oxide in the crystal grainboundary and the periphery thereof, offering chemical and structuralstability to the positive electrode active material particle, so thatdeterioration of the positive electrode active material, such asdissolution of the transition metal to an electrolyte solution, releaseof oxygen, and unstable crystal structure, can be inhibited. Inaddition, cracking of the positive electrode active material particlecan be inhibited. Release of oxygen from the positive electrode activematerial particle can also be inhibited. The use of such a positiveelectrode active material particle can inhibit deterioration of a powerstorage device. In addition, a highly safe power storage device can beachieved. When the charge voltage is increased, the amount of lithiumincluded in a positive electrode is reduced when charging, and thecrystal structure of a positive electrode active material particle isprone to change; thus, Sample A is particularly preferable as thepositive electrode active material particle.

FIG. 41(B) shows the ratio of the fluorine atomic concentration to theatomic concentration of the transition metal (Tr-Metal) in the crystalgrain. In FIG. 41(B), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the fluorine atomicconcentration to the atomic concentration of the transition metal in thecrystal grain (F/Tr-Metal).

As shown in FIG. 41(B), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the fluorineatomic concentration to the atomic concentration of the transition metal(F/Tr-Metal) in the crystal grain is greater than or equal to 0.030.Fluorine in the crystal grain boundary and the periphery thereof wasfound to contribute to efficient segregation of magnesium in the crystalgrain boundary and the periphery thereof.

FIG. 41(C) shows the ratio of the magnesium (Mg) atomic concentration tothe total atomic concentration of nickel, manganese, and cobalt(Ni+Mn+Co) at each measurement point of EDX. In FIG. 41(C), thehorizontal axis represents the distance [nm] and the vertical axisrepresents the ratio of the magnesium atomic concentration to the totalatomic concentration of nickel, manganese, and cobalt (Mg/(Ni+Mn+Co)) ateach measurement point of EDX.

The total atomic concentration of nickel, manganese, and cobalt(Ni+Mn+Co) at each measurement point of EDX is the same as that in thedata shown in FIG. 40(F).

As shown in FIG. 41(C), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the total atomic concentration of nickel,manganese, and cobalt (Mg(Ni+Mn+Co)) in the crystal grain is greaterthan or equal to 0.030. Magnesium was found to be segregated in thecrystal grain boundary and the periphery thereof.

FIG. 41(D) shows the ratio of the fluorine atomic concentration to thetotal atomic concentration of nickel, manganese, and cobalt (Ni+Mn+Co)at each measurement point of EDX. In FIG. 41(D), the horizontal axisrepresents the distance [nm] and the vertical axis represents the ratioof the fluorine atomic concentration to the total atomic concentrationof nickel, manganese, and cobalt (F/(Ni+Mn+Co)) at each measurementpoint of EDX.

As shown in FIG. 41(D), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the fluorineatomic concentration to the atomic concentration of the transition metal(F/(Ni+Mn+Co)) in the crystal grain is greater than or equal to 0.030.Fluorine in the crystal grain boundary and the periphery thereof wasfound to contribute to efficient segregation of magnesium in the crystalgrain boundary and the periphery thereof.

This example revealed that by adding magnesium and fluorine as startingmaterials of a positive electrode active material particle, magnesium issegregated in a crystal grain boundary and the periphery thereof in thepositive electrode active material particle. The positive electrodeactive material particle of one embodiment of the present invention,which includes magnesium oxide in the crystal grain boundary, ischemically and structurally stable and hardly undergoes a change instructure, a change in volume, and strain due to charge and discharge.In other words, the crystal structure of the positive electrode activematerial particle is more stable and hardly changes even afterrepetition of charge and discharge. In addition, cracking of thepositive electrode active material particle can be inhibited. That is,deterioration such as a reduction in capacity can be reduced.

A power storage device including such a positive electrode activematerial particle is unlikely to deteriorate and thus is suitable for aportable electronic device. Furthermore, when used to cars and othervehicles, it is also possible to avoid using commercial power at thepeak of electric power demand, which can contribute to energy saving andreduction of carbon dioxide emissions. In addition, a highly safe powerstorage device is achieved.

Example 2

In this example, a positive electrode active material particle includingmagnesium, fluorine, and oxygen in a crystal grain boundary and theperiphery thereof was fabricated and the concentration distribution in acrystal grain and a crystal grain boundary in the active material wasfound by TEM observation and STEM-EDX analysis. Sample B was prepared asa sample of one embodiment of the present invention. As Sample B,lithium cobalt oxide including magnesium, fluorine, and oxygen in acrystal grain boundary and the periphery thereof was fabricated. Lithiumcobalt oxide was assumed to have a composition of LiCoO₂. LiCoO₂ has alayered rock-salt crystal structure.

<Fabrication of Sample B>

The fabrication of Sample B will be described.

Starting materials were prepared as shown in Step S11 in the flowchartof FIG. 3. Lithium carbonate (Li₂CO₃) as a lithium source, tricobalttetroxide (Co₃O₄) as a cobalt source, magnesium oxide (MgO) as amagnesium source, and lithium fluoride (LiF) as a fluorine source wereweighed. Specifically, 3.1489 g (42.62 mmol) of Li₂CO₃, 6.7726 g (28.13mmol) of Co₃O₄, 0.0344 g (0.85 mmol) of MgO, and 0.0442 g (1.70 mmol) ofLiF were weighed. According to this, the ratio m of the number ofmagnesium atoms to the number of cobalt atoms is 0.010 (1.0%). Inaddition, the ratio n of the number of fluorine atoms to the number ofmagnesium atoms is 2.0. Note that Li₂CO₃ used is a product of KojundoChemical Laboratory Co., Ltd. (catalog No. LIH06XB). MgO used is aproduct of Kojundo Chemical Laboratory Co., Ltd. (catalog No. MGO12PB).LiF used is a product of Kojundo Chemical Laboratory Co., Ltd. (catalogNo. LIH10XB).

Next, as shown in Step S12, the starting materials weighed in Step S11were mixed. For the details of the mixing, the description on Sample Acan be referred to, and thus the description is omitted here.

Next, as shown in Step S13, a first heating was performed on thematerials mixed in Step S12. For the details of the first heating, thedescription on Sample A can be referred to, and thus the description isomitted here.

Next, as shown in Step S14, the materials heated in Step S13 were cooledto room temperature to obtain a synthetic material 2. After the cooling,the synthetic material 2 was subjected to crushing treatment, wherebythe particle size of the synthetic material 2 was reduced. A 53-μm meshwas used for the crushing treatment.

Next, as shown in Step S15, a second heating was performed on thesynthetic material 2 obtained in Step S14. For the details of the secondheating, the description on Sample A can be referred to, and thus thedescription is omitted here.

The second heating in Step S15 promotes segregation of the magnesium andfluorine contained in the starting materials into the crystal grainboundary of lithium cobalt oxide.

Next, as shown in Step S16, the synthetic material 2 heated in Step S15was cooled to room temperature and collected, so that Sample B wasobtained.

<TEM Observation. STEM Observation, and EDX Measurement>

Then, Sample B was thinned by focused ion beam (FIB) and a cross sectionof Sample B was observed with TEM and STEM. Furthermore, the compositionanalysis of the cross section of Sample B was performed by EDXmeasurement. For the details of the TEM and STEM observation and the EDXmeasurement, the description on Sample A can be referred to, and thusthe description is omitted here.

FIG. 42(A) shows a cross-sectional TEM image (a bright-field image) ofSample B. The magnification of FIG. 42(A) is 100,000 times. In FIG.42(A), a region where the concentration (luminance) of the TEM image issubstantially uniform probably has a substantially uniform crystalorientation, i.e., a single crystal. A region where the concentration(luminance) of the TEM image changes is probably a grain boundary. FIG.42(B) shows a schematic diagram corresponding to FIG. 42(A). As shown inFIG. 42(A) and FIG. 42(B), the positive electrode active materialparticle was found to include a crystal grain boundary 1203 between aplurality of crystal grains 1201 and a crystal grain.

FIG. 43(A) shows a cross-sectional STEM image (a bright-field image) ofSample B, and FIG. 43(B) shows a HAADF-STEM image of the same point. Themagnification of FIG. 43(A) and FIG. 43(B) is 8,000.000 times. A crystallattice image was observed in a crystal grain region in FIG. 43(A) andFIG. 43(B).

FIG. 44(A) shows a HAADF-STEM image of the region of Sample B that wassubjected to the EDX plane analysis. The EDX plane analysis wasperformed in a region including a crystal grain and a crystal grainboundary. In this example, the EDX measurement was performed on 256×256points in the region.

The peaks derived from electron transition to the K shell in carbon,oxygen, fluorine, magnesium, silicon, phosphorus, sulfur, calcium,manganese, cobalt, and nickel were observed. The obtained spectra wereseparated into those of the respective elements, so that the atomicconcentrations were obtained.

FIG. 44(B) shows a mapping image of carbon in the EDX plane analysis ofthe region illustrated in FIG. 44(A); FIG. 44(C), oxygen; FIG. 44(D),fluorine; FIG. 44(E), magnesium; FIG. 44(F), silicon; FIG. 45(A),phosphorus; FIG. 45(B), sulfur; FIG. 45(C), calcium; and FIG. 45(D),cobalt.

FIG. 44(B) to FIG. 44(F) and FIG. 45(A) to FIG. 45(D) each show theintensity mapping of characteristic X-ray obtained by the EDXmeasurement; a measurement point with a low characteristic X-rayintensity is denoted with a pale color (white), and a measurement pointwith a higher characteristic X-ray intensity is denoted with a darkercolor (black). In other words, the pale color (white) measurement pointmeans a low atomic concentration whereas the dark color (black)measurement point means a high atomic concentration. Note that in FIG.44(B) to FIG. 44(F) and FIG. 45(A) to FIG. 45(D), the scale of thecharacteristic X-ray intensity differs for each element so as to clearlyshow the distribution in the region.

As shown in FIG. 44(B) to FIG. 44(F) and FIG. 45(A) to FIG. 45(D), theconcentrations of magnesium and calcium were found to be high in thecrystal grain boundary and the periphery thereof. Almost no fluorine wasobserved in the region subjected to the EDX plane analysis. This isprobably because EDX is hard to detect fluorine which is a lightweightelement. Note that calcium was probably contained in a reagent used as araw material.

Data in linear regions was extracted from the EDX plane analysis shownin FIG. 44(B) to FIG. 44(F) and FIG. 45(A) to FIG. 45(D), and thedistribution of the atomic concentrations in the positive electrodeactive material particle was evaluated.

FIG. 46(A) shows a HAADF-STEM image of the region of Sample B that wassubjected to the EDX linear analysis. In FIG. 46(A), the regionsubjected to the EDX linear analysis is denoted by an arrow. The EDXlinear analysis was performed on a crystal grain, a crystal grainboundary, and a region across the crystal grain.

FIG. 47(A) shows the atomic concentration of carbon in the EDX linearanalysis of the region illustrated in FIG. 46(A); FIG. 47(B), oxygen;FIG. 47(C), fluorine; FIG. 47(D), magnesium; FIG. 47(E), silicon; FIG.47(F), phosphorus; FIG. 48(A), sulfur; FIG. 48(B), calcium; and FIG.48(C), cobalt.

In FIG. 47(A) to FIG. 47(F) and FIG. 48(A) to FIG. 48(C), the horizontalaxis represents the distance [nm] and the vertical axis represents theatomic concentration [atomic %]. The distance on the horizontal axis isshown so as to increase from the starting point (distance=0 nm), whichis indicated as a black dot on one end of the arrow illustrated in FIG.46(A), to the other end (ending portion). The atomic concentration onthe vertical axis shows the percentage of the number of atoms for eachelement with respect to the total number of atoms of carbon, oxygen,fluorine, magnesium, silicon, phosphorus, sulfur, calcium, and cobalt as100 atomic %.

As shown in FIG. 46(A), FIG. 47(A) to FIG. 47(F), and FIG. 48(A) to FIG.48(C), the concentrations of magnesium and calcium were found to behigher in the crystal grain boundary and the periphery thereof than inthe crystal grain region. It was also found that the crystal grainboundary and the periphery thereof had a region with a width greaterthan or equal to 1 nm and less than or equal to 10 nm.

The crystal grain boundary and the periphery thereof were found toinclude oxygen and magnesium. The crystal grain boundary and theperiphery thereof were found to include magnesium oxide.

In contrast, fluorine, magnesium, silicon, and calcium were at the levelof the lower detection limit in the crystal grain region.

Phosphorus and sulfur were at the level of the lower detection limit inthe crystal grain and the crystal grain boundary.

The carbon concentration detected in the crystal grain and the crystalgrain boundary probably includes carbon derived from a carbon coat filmused as a protective film. It was thus not possible to determine theactual carbon concentration in the crystal grain and the crystal grainboundary.

The atomic concentration of cobalt, which is a transition metal, wasfound to be lower in the crystal grain boundary and the peripherythereof than in the crystal grain.

The atomic concentration of cobalt in Sample B can be regarded as theatomic concentration of the transition metal. As shown in FIG. 48(C),the atomic concentration of the transition metal was found to be proneto be lower in the crystal grain boundary and the periphery thereof thanthat in the crystal grain region. It was also found that the atomicconcentration of the transition metal in the crystal grain region wassubstantially uniform without large variation.

FIG. 49(A) shows the ratio of the atomic concentration of magnesium (Mg)to the atomic concentration of the transition metal in the crystalgrain. In FIG. 49(A), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the atomic concentrationof magnesium to the atomic concentration of the transition metal in thecrystal grain (Mg/Tr-Metal).

The average atomic concentration of the transition metals in the crystalgrain was used as the atomic concentration of the transition metal(Tr-Metal) in the crystal grain. The crystal grain region used for thecalculation of the average value is indicated by arrows in FIG. 48(D).

As shown in FIG. 49(A), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the atomic concentration of the transition metal(Mg/Tr-Metal) in the crystal grain is greater than or equal to 0.030.Magnesium was found to be segregated in the crystal grain boundary andthe periphery thereof. The crystal grain boundary and the peripherythereof probably include magnesium oxide. Sample B of one embodiment ofthe present invention includes magnesium oxide in the crystal grainboundary and the periphery thereof, offering chemical and structuralstability to the positive electrode active material particle, so thatdeterioration of the positive electrode active material, such asdissolution of the transition metal to an electrolyte solution, releaseof oxygen, and unstable crystal structure, can be inhibited. Inaddition, cracking of the positive electrode active material particlecan be inhibited. Release of oxygen from the positive electrode activematerial particle can also be inhibited. The use of such a positiveelectrode active material particle can inhibit deterioration of a powerstorage device. In addition, a highly safe power storage device can beachieved. When the charge voltage is increased, the amount of lithiumincluded in a positive electrode is reduced when charging, and thecrystal structure of a positive electrode active material particle isprone to change; thus, Sample B is particularly preferable as thepositive electrode active material particle.

FIG. 49(B) shows the ratio of the fluorine atomic concentration to theatomic concentration of the transition metal (Tr-Metal) in the crystalgrain. In FIG. 49(B), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the fluorine atomicconcentration to the atomic concentration of the transition metal in thecrystal grain (F/Tr-Metal).

As shown in FIG. 47(C) and FIG. 49(B), the fluorine concentration inSample B was at the level of the lower detection limit in the crystalgrain and the crystal grain boundary. This is probably because EDX ishard to detect fluorine which is a lightweight element.

FIG. 49(C) shows the ratio of the magnesium (Mg) atomic concentration tothe cobalt (Co) atomic concentration at each measurement point of EDX.In FIG. 49(C), the horizontal axis represents the distance [nm] and thevertical axis represents the ratio of the magnesium atomic concentrationto the cobalt atomic concentration (Mg/Co) at each measurement point ofEDX.

As shown in FIG. 49(C), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the cobalt atomic concentration (Mg/Co) in thecrystal grain is greater than or equal to 0.030. Magnesium was found tobe segregated in the crystal grain boundary and the periphery thereof.

FIG. 49(D) shows the ratio of the fluorine atomic concentration to thecobalt (Co) atomic concentration at each measurement point of EDX. InFIG. 49(D), the horizontal axis represents the distance [nm] and thevertical axis represents the ratio of the fluorine atomic concentrationto the cobalt atomic concentration (F/Co) at each measurement point ofEDX. The fluorine concentration in Sample B was at the level of thelower detection limit in the crystal grain and the crystal grainboundary.

Similar EDX measurement was performed on another portion of Sample B.

FIG. 50(A) shows a HAADF-STEM image of the region of Sample B that wassubjected to the EDX plane analysis. The EDX plane analysis wasperformed in a region including a crystal grain and a crystal grainboundary. FIG. 50(B) shows a mapping image of carbon in the EDX planeanalysis of the region illustrated in FIG. 50(A); FIG. 50(C), oxygenFIG. 50(D), fluorine; FIG. 50(E), magnesium; FIG. 50(F), silicon; FIG.51(A), phosphorus; FIG. 51(B), sulfur; FIG. 51(C), calcium; and FIG.51(D), cobalt.

FIG. 50(B) to FIG. 50(F) and FIG. 51(A) to FIG. 51(D) each show theintensity mapping of characteristic X-ray obtained by the EDXmeasurement; a measurement point with a low characteristic X-rayintensity is denoted with a pale color (white), and a measurement pointwith a higher characteristic X-ray intensity is denoted with a darkercolor (black). In other words, the pale color (white) measurement pointmeans a low atomic concentration whereas the dark color (black)measurement point means a high atomic concentration. Note that in FIG.50(B) to FIG. 50(F) and FIG. 51(A) to FIG. 51(D), the scale of thecharacteristic X-ray intensity differs for each element so as to clearlyshow the distribution in the region.

As shown in FIG. 50(B) to FIG. 50(F) and FIG. 51(A) to FIG. 51(D), theconcentrations of magnesium and calcium were found to be high in thecrystal grain boundary and the periphery thereof. Almost no fluorine wasobserved in the region subjected to the EDX plane analysis. This isprobably because EDX is hard to detect fluorine which is a lightweightelement. Note that calcium was probably contained in a reagent used as araw material.

Data in linear regions was extracted from the EDX plane analysis shownin FIG. 50(B) to FIG. 50(F) and FIG. 51(A) to FIG. 51(D), and thedistribution of the atomic concentrations in the positive electrodeactive material particle was evaluated.

FIG. 46(B) shows a HAADF-STEM image of the region of Sample B that wassubjected to the EDX linear analysis. In FIG. 46(B), the regionsubjected to the EDX linear analysis is denoted by an arrow. The EDXlinear analysis was performed on a crystal grain, a crystal grainboundary, and a region across the crystal grain.

FIG. 52(A) shows the atomic concentration of carbon in the EDX linearanalysis of the region illustrated in FIG. 46(B); FIG. 52(B), oxygen;FIG. 52(C), fluorine; FIG. 52(D), magnesium; FIG. 52(E), silicon; FIG.52(F), phosphorus; FIG. 53(A), sulfur; FIG. 53(B), calcium; and FIG.53(C), cobalt.

In FIG. 52(A) to FIG. 52(F) and FIG. 53(A) to FIG. 53(C), the horizontalaxis represents the distance [nm] and the vertical axis represents theatomic concentration [atomic %]. The distance on the horizontal axis isshown so as to increase from the starting point (distance=0 nm), whichis indicated as a black dot on one end of the arrow illustrated in FIG.46(B), to the other end (ending portion). The atomic concentration onthe vertical axis shows the percentage of the number of atoms for eachelement with respect to the total number of atoms of carbon, oxygen,fluorine, magnesium, silicon, phosphorus, sulfur, calcium, and cobalt as100 atomic/o.

As shown in FIG. 46(B), FIG. 52(A) to FIG. 52(F), and FIG. 53(A) to FIG.53(C), the concentration of magnesium was found to be higher in thecrystal grain boundary and the periphery thereof than in the crystalgrain region. It was also found that the crystal grain boundary and theperiphery thereof had a region with a width greater than or equal to 1nm and less than or equal to 10 nm.

The crystal grain boundary and the periphery thereof were found toinclude oxygen and magnesium. The crystal grain boundary and theperiphery thereof were found to include magnesium oxide.

In contrast, fluorine, magnesium, silicon, and calcium were at the levelof the lower detection limit in the crystal grain region.

Phosphorus and sulfur were at the level of the lower detection limit inthe crystal grain and the crystal grain boundary.

The carbon concentration detected in the crystal grain and the crystalgrain boundary probably includes carbon derived from a carbon coat filmused as a protective film. It was thus not possible to determine theactual carbon concentration in the crystal grain and the crystal grainboundary.

The atomic concentration of cobalt, which is a transition metal, wasfound to be lower in the crystal grain boundary and the peripherythereof than in the crystal grain.

The atomic concentration of cobalt in Sample B can be regarded as theatomic concentration of the transition metal. As shown in FIG. 53(C),the atomic concentration of the transition metal was found to be proneto be lower in the crystal grain boundary and the periphery thereof thanthat in the crystal grain region. It was also found that the atomicconcentration of the transition metal in the crystal grain region wassubstantially uniform without large variation.

FIG. 54(A) shows the ratio of the atomic concentration of magnesium (Mg)to the atomic concentration of the transition metal in the crystalgrain. In FIG. 54(A), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the atomic concentrationof magnesium to the atomic concentration of the transition metal in thecrystal grain (Mg/Tr-Metal).

The average atomic concentration of the transition metals in the crystalgrain was used as the atomic concentration of the transition metal(Tr-Metal) in the crystal grain. The crystal grain region used for thecalculation of the average value is indicated by arrows in FIG. 53(D).

As shown in FIG. 54(A), the crystal grain boundary and the peripherythereof were found to include a region there the ratio of the magnesiumatomic concentration to the atomic concentration of the transition metal(Mg/Tr-Metal) in the crystal grain is greater than or equal to 0.030.Magnesium was found to be segregated in the crystal grain boundary andthe periphery thereof. The crystal grain boundary and the peripherythereof probably include magnesium oxide. Sample B of one embodiment ofthe present invention includes magnesium oxide in the crystal grainboundary and the periphery thereof, offering chemical and structuralstability to the positive electrode active material particle, so thatdeterioration of the positive electrode active material, such asdissolution of the transition metal to an electrolyte solution, releaseof oxygen, and unstable crystal structure, can be inhibited. Inaddition, cracking of the positive electrode active material particlecan be inhibited. Release of oxygen from the positive electrode activematerial particle can also be inhibited. The use of such a positiveelectrode active material particle can inhibit deterioration of a powerstorage device. In addition, a highly safe power storage device can beachieved. When the charge voltage is increased, the crystal structure ofa positive electrode active material particle is prone to change; thus,Sample B is particularly preferable as the positive electrode activematerial particle.

FIG. 54(B) shows the ratio of the fluorine atomic concentration to theatomic concentration of the transition metal (Tr-Metal) in the crystalgrain. In FIG. 54(B), the horizontal axis represents the distance [nm]and the vertical axis represents the ratio of the fluorine atomicconcentration to the atomic concentration of the transition metal in thecrystal grain (F/Tr-Metal).

As shown in FIG. 52(C) and FIG. 54(B), the fluorine concentration inSample B was at the level of the lower detection limit in the crystalgrain and the crystal grain boundary. This is probably because EDX ishard to detect fluorine which is a lightweight element.

FIG. 54(C) shows the ratio of the magnesium (Mg) atomic concentration tothe cobalt (Co) atomic concentration at each measurement point of EDX.In FIG. 54(C), the horizontal axis represents the distance [nm] and thevertical axis represents the ratio of the magnesium atomic concentrationto the cobalt atomic concentration (Mg/Co) at each measurement point ofEDX.

As shown in FIG. 54(C), the crystal grain boundary and the peripherythereof were found to include a region where the ratio of the magnesiumatomic concentration to the cobalt atomic concentration (Mg/Co) in thecrystal grain is greater than or equal to 0.030. Magnesium was found tobe segregated in the crystal grain boundary and the periphery thereof.

FIG. 54(D) shows the ratio of the fluorine atomic concentration to thecobalt (Co) atomic concentration at each measurement point of EDX. InFIG. 54(D), the horizontal axis represents the distance [nm] and thevertical axis represents the ratio of the fluorine atomic concentrationto the cobalt atomic concentration (F/Co) at each measurement point ofEDX. The fluorine concentration in Sample B was at the level of thelower detection limit in the crystal grain and the crystal grainboundary.

REFERENCE NUMERALS

100: positive electrode active material particle, 101: crystal grain,103: crystal grain boundary, 105: crystal defect, 107: region, 200:active material layer, 201: graphene compound, 211 a: positiveelectrode, 211 b: negative electrode, 212 a: lead, 212 b: lead, 214:separator, 215 a: bonding portion, 215 b: bonding portion, 217: fixingmember, 250: battery. 251: exterior body, 261: folded portion, 262: sealportion 263: seal portion, 271: crest line, 272: trough line, 273:space, 300: secondary battery, 301: positive electrode can, 302:negative electrode can, 303: gasket, 304: positive electrode, 305:positive electrode current collector. 306: positive electrode activematerial layer, 307: negative electrode, 308: negative electrode currentcollector, 309: negative electrode active material layer, 310:separator, 500: secondary battery, 501: positive electrode currentcollector, 502: positive electrode active material layer. 503: positiveelectrode, 504: negative electrode current collector, 505: negativeelectrode active material layer, 506: negative electrode, 507:separator, 508: electrolyte solution, 509: exterior body, 510: positiveelectrode lead electrode, 511: negative electrode lead electrode, 600:secondary battery, 601: positive electrode cap, 602: battery can, 603:positive electrode terminal, 604: positive electrode, 605: separator,606: negative electrode, 607: negative electrode terminal, 608:insulating plate, 609: insulating plate, 611: PTC element, 612: safetyvalve mechanism. 900: circuit board, 910: label, 911: terminal, 912:circuit. 913: secondary battery. 914: antenna, 915: antenna, 916: layer,917: layer, 918: antenna, 919: terminal, 920: display device, 921:sensor, 922: terminal, 930: housing, 930 a: housing, 930 b: housing,931: negative electrode. 932: positive electrode, 933: separator, 950:wound body, 951: terminal, 952: terminal, 980: secondary battery. 993:wound body, 994: negative electrode, 995: positive electrode, 996:separator, 997: lead electrode. 998: lead electrode, 1101: crystalgrain, 1103: crystal grain boundary, 1201: crystal grain, 1203: crystalgrain boundary, 7100: portable display device, 7101: housing, 7102:display portion, 7103: operation button, 7104: secondary battery, 7200:portable information terminal, 7201: housing, 7202: display portion,7203: band. 7204: buckle, 7205: operation button, 7206: input outputterminal, 7207: icon, 7300: display device. 7304: display portion, 7400:mobile phone, 7401: housing, 7402: display portion, 7403: operationbutton, 7404: external connection port, 7405: speaker, 7406: microphone,7407: secondary battery. 8000: display device, 8001: housing. 8002:display portion, 8003: speaker portion, 8004: secondary battery, 8021:charging apparatus, 8022: cable, 8024: secondary battery, 8100: lightingdevice, 8101: housing, 8102: light source, 8103: secondary battery,8104: ceiling. 8105: sidewall, 8106: floor, 8107: window, 8200: indoorunit, 8201: housing. 8202: air outlet, 8203: secondary battery, 8204:outdoor unit, 8300: electric refrigerator-freezer, 8301: housing. 8302:refrigerator door, 8303: freezer door, 8304: secondary battery, 8400:automobile, 8401: headlight, 8406: electric motor, 8500: automobile,8600: motor scooter, 8601: side mirror, 8602: secondary battery, 8603:indicator, 8604: storage unit under seat, 9600: tablet terminal, 9625:switch, 9626: switch, 9627: power switch, 9628: operation switch, 9629:fastener, 9630: housing, 9630 a: housing, 9630 b: housing, 9631: displayportion. 9633: solar cell, 9634: charge and discharge control circuit,9635: power storage unit. 9636: DC-DC converter, 9637: converter, 9640:movable portion.

The invention claimed is:
 1. A lithium-ion secondary battery comprising:a positive electrode active material particle including a crystal grainboundary and a plurality of crystal grains, wherein the positiveelectrode active material particle comprises a region in which a ratioof a number of magnesium atoms in the crystal grain boundary or aperiphery of the crystal grain boundary to a number of cobalt atoms inone of the plurality of the crystal grains is greater than or equal to0.010 and less than or equal to 0.5.
 2. The lithium-ion batteryaccording to claim 1, wherein the positive electrode active materialparticle comprises a region in which a ratio of a number of magnesiumatoms in the crystal grain boundary or the periphery of the crystalgrain boundary to a number of cobalt atoms in the one of the pluralityof the crystal grains is greater than or equal to 0.03.
 3. Thelithium-ion secondary battery according to claim 1, wherein, in EDXlinear analysis for the one of the plurality of the crystal grains,magnesium is detected at a level of the lower detection limit, andwherein, in EDX linear analysis for the crystal grain boundary or theperiphery of the crystal grain boundary, magnesium is detected beyondthe level of the lower detection limit.
 4. The lithium-ion secondarybattery according to claim 1, wherein, in EDX linear analysis for theone of the plurality of the crystal grains, magnesium is detected atlower than 1 atomic %, and wherein, in EDX linear analysis for thecrystal grain boundary or the periphery of the crystal grain boundary,magnesium is detected beyond 1 atomic %.
 5. The lithium-ion secondarybattery according to claim 1, wherein the number of magnesium atoms andthe number of cobalt atoms is obtained with EDX linear analysis.
 6. Thelithium-ion battery according to claim 1, further comprising aconductive additive, wherein the conductive additive includes carbonfiber.
 7. The lithium-ion battery according to claim 6, wherein thecarbon fiber includes carbon nanofiber or carbon nanotube.
 8. Thelithium-ion battery according to claim 6, wherein the conductiveadditive further includes any one of carbon black, graphite particle,graphene, and fullerene.
 9. The lithium-ion battery according to claim1, wherein the positive electrode active material particle comprises aregion in which a ratio of a number of fluorine atoms in the crystalgrain boundary or a periphery of the crystal grain boundary to a numberof cobalt atoms in one of the plurality of the crystal grains is greaterthan or equal to 0.020 and less than or equal to 1.00.
 10. A lithium-ionsecondary battery comprising: a positive electrode active materialparticle including a crystal grain boundary and a plurality of crystalgrains, wherein the positive electrode active material particlecomprises a region in which a ratio of a number of magnesium atoms inthe crystal grain boundary or a periphery of the crystal grain boundaryto a number of transition metal atoms in one of the plurality of thecrystal grains is greater than or equal to 0.010 and less than or equalto 0.5, wherein the transition metal comprises cobalt, nickel andmanganese, and wherein the number of the transition metal atoms refersto the total number of atoms of each of cobalt, nickel, and manganeseincluded in the one of the plurality of the crystal grains.
 11. Thelithium-ion battery according to claim 10, wherein the positiveelectrode active material particle comprises a region in which a ratioof a number of magnesium atoms in the crystal grain boundary or theperiphery of the crystal grain boundary to a number of transition metalatoms in the one of the plurality of the crystal grains is greater thanor equal 0.03.
 12. The lithium-ion secondary battery according to claim10, wherein, in EDX linear analysis for the one of the plurality of thecrystal grains, magnesium is detected at a level of the lower detectionlimit, and wherein, in EDX linear analysis for the crystal grainboundary or the periphery of the crystal grain boundary, magnesium isdetected beyond the level of the lower detection limit.
 13. Thelithium-ion secondary battery according to claim 10, wherein, in EDXlinear analysis for the one of the plurality of the crystal grains,magnesium is detected at lower than 1 atomic %, and wherein, in EDXlinear analysis for the crystal grain boundary or the periphery of thecrystal grain boundary, magnesium is detected beyond 1 atomic %.
 14. Thelithium-ion secondary battery according to claim 10, wherein the numberof magnesium atoms and the number of transition metal atoms is obtainedwith EDX linear analysis.
 15. The lithium-ion battery according to claim10, further comprising a conductive additive, wherein the conductiveadditive includes carbon fiber.
 16. The lithium-ion battery according toclaim 15, wherein the carbon fiber includes carbon nanofiber or carbonnanotube.
 17. The lithium-ion battery according to claim 15, wherein theconductive additive further includes any one of carbon black, graphiteparticle, graphene, and fullerene.